In HeartGold, Caterpie and Metapod are available on Routes 2, 30 and 31, as well as the Ilex Forest, National Park, and Viridian Forest. They only appear in the morning or during the day. In both games Caterpie, Metapod and Butterfree can be found in the Bug-Catching Contest.
Compoundeyes and Sleep Powder are all the introduction Butterfree will ever need. With its main selling point being status moves with near-perfect accuracy, Butterfree is an absolute monster in the early game, after its evolution completes. Naturally, as most bugs do, it loses steam fairly quickly starting from the mid-game onwards, which makes it nearly impossible for it to be of any use in later matchups; however, even with its relatively limited Speed, Butterfree can rack up a number of wins even just by sleep-subbing alone, or at the very least provide some team support, or even just be a general top tier catcher. Still, players should not expect Butterfree to shine for very long.
Gym #1 - Falkner (Violet City, Flying-type): Butterfree can dispatch Pidgey easily with two Confusions, but is at a serious disadvantage against Pidgeotto. Depending on whether or not it can outspeed, the battle may be dangerous. Butterfree may either use PoisonPowder and pass the battle to a teammate that can take Gust well, as the move easily 3HKOs Butterfree, or use Stun Spore and hope to nail it with enough Confusion hits before it deals too much damage, likely stalling with some potions throughout.
Proton (Slowpoke Well): One Confusion for Zubat and two for Koffing slap them both right into next week.
Gym #2 - Bugsy (Azalea Town, Bug-type): Butterfree's presence here can be anywhere from very helpful to rather dangerous for its safety. Kakuna and Metapod are hardly an issue; Scyther's Quick Attack, just like Butterfree's Gust, is a 3HKO, but for Scyther it is a priority move, and it tends to Leer and use Focus Energy to hone its killing potential. To be successful, Butterfree can either shoot for Sleep Powder and attack with Gust until Scyther is down, hoping that the sleep turns last long enough for that to happen, or put Scyther to sleep and then spam Harden to improve its survival rate in absence of critical hits. As Quick Attack is a 3HKO normally, a critical Quick Attack at non-negative Defense modifiers will deal roughly 70%, meaning that it is also possible to heal stall. If Scyther somehow uses U-turn instead, Butterfree can reach +6 Defense with Harden while fighting Metapod or Kakuna, and then take down Scyther with ease.
Rival (Azalea Town): Confusion will easily blast Gastly and Zubat into oblivion. Bayleef only really has PoisonPowder to deal damage, since Razor Leaf will barely tickle Butterfree, as well as being weak to Gust. Croconaw is harder to beat, due to Butterfree's poor Defense and its high Attack, but Butterfree should manage if it can put it to sleep with Sleep Powder and then whittle down its health; Gust 4HKOs and Croconaw's Bite 3HKOs, so two sleep turns will be sufficient for Butterfree to win. Quilava is even worse due to STAB super effective Ember, though a healthy Butterfree can survive one hit and put it to sleep before switching out.
Gym #3 - Whitney (Goldenrod City, Normal-type): Butterfree can beat Clefairy without taking damage with Sleep Powder and Gust. A Wide Lens, obtainable at the Game Corner, will boost Sleep Powder's accuracy to over 100%, and Butterfree can complement this with Substitute. However, how well Butterfree will do against Miltank depends on its Speed; it is more than likely that Butterfree will not be able to outspeed Miltank, unless it has a beneficial nature and plenty of Speed EVs. The only way Butterfree can win if it does not outspeed is by having a Substitute up after Clefairy falls, and at least 70% of total health remaining. Butterfree needs to Sleep Powder on the first turn, using up Miltank's Lum Berry, and its predictable Rollout will take down the sub. Protect can then break the streak - necessary, as a stage 2 Rollout will easily kill Butterfree - and Butterfree can take a stage 1 Rollout before putting Miltank to sleep again, but this is assuming Rollout will not score a critical hit. Butterfree can then keep on using Substitute and Sleep Powder, alternating with Gust damage, to take Miltank down. it is a VERY dangerous strategy, though: one critical hit will off Butterfree for good.
Rival (Burned Tower): Confusion easily defeats Gastly and Zubat; the latter will take a hit before going down, but Butterfree can tank its Wing Attack in return. Magnemite resists Butterfree's attacks, and can hit it with super effective ThunderShock, but Butterfree can get the best of it with a combination of Sleep Powder and Substitute, or Sleep Powder and some luck. Bayleef is easy to defeat, but both Quilava and Croconaw have super effective physical moves - the latter in the form of Ice Fang - which Butterfree will find very hard to take, so once again, it needs to resort to Sleep Powder and Substitute or bail out of the fight completely.
Gym #4 - Morty (Ecruteak City, Ghost-type): Butterfree's good Speed should be sufficient to surpass Gastly's, allowing it to repeat the Whitney strategy: Sleep Powder first, then Substitute, then Psybeam spam. Psybeam is an OHKO against Gastly and a 2HKO against Haunter, but it is best for Butterfree to put Gastly to sleep either way, so as to attempt a Substitute before the first Haunter comes; bear in mind, however, that Curse bypasses the Substitute, so Morty's Pokémon will likely force Butterfree to switch out at some point. Whether or not Butterfree is safe enough to fight Gengar depends on the presence or absence of a Substitute before Morty's ace comes into play; if it can be put to sleep and Butterfree manages to keep the Substitute, replenishing HP when needed, Butterfree can win, but otherwise Shadow Ball will outdamage Butterfree's Psybeam and Gengar also outspeeds, making the fight very risky if Mean Look is also involved. It is strongly suggested that Butterfree be switched out as soon as the Substitute breaks, to prevent any unnecessary risks.
Eusine (Cianwood City): Butterfree can raid Drowzee with Silver Wind if it already has it, and then defeat Haunter with Psybeam. Electrode has Thunder, so Butterfree needs to switch out against it.
Gym #5 - Chuck (Cianwood City, Fighting-type): Primeape is a horrible opponent for Butterfree, because it knows Rock Slide and has the ability Vital Spirit, which prevents Butterfree from putting it to sleep. Poliwrath is better, but still difficult; Butterfree's Psybeam deals a consistently lesser amount of damage than Poliwrath's Body Slam and Surf, though even a critical hit from either move cannot kill Butterfree. Still, Butterfree is not particularly afraid of Hypnosis as it doubly resists Fighting moves, so it can overcome Poliwrath with, at worst, several rounds of heal-stalling.
Gym #6 - Jasmine (Olivine City, Steel-type): While Butterfree's Sleep Powder may be tempting to use, there is not much else it can do in either of the fights, and all of Jasmine's Pokémon are dangerous, sporting respectively Thunderbolt (the two Magnemite) and Rock Throw (Steelix). It is advised to just put them to sleep and switch, or to not use Butterfree at all.
Petrel (Team Rocket HQ): Both Zubat and Koffing will go down to a single Psybeam, whereas Raticate can be 2HKOed by Silver Wind.
Ariana (Team Rocket HQ, tag battle with Lance): A Butterfree with Psybeam and Silver Wind will have little to no issue against anything but Murkrow which, however, is a major threat. It has both Wing Attack and Pursuit, neither of which is enjoyable for Butterfree's low Defense, as well as being faster than it. Even switching out is ill-advised, so it is recommended that Butterfree leaves the battle before Murkrow comes out, possibly disposing of Drowzee and Grimer before doing so, so that Murkrow will be the very last one left on the field when Gloom goes down.
Gym #7 - Pryce (Mahogany Town, Ice-type): Seel can be 3HKOed by Silver Wind, but other than that, this is no place for Butterfree to be. Dewgong has Aurora Beam, which 2HKOs, and Ice Shard, which does not do much less than that; Piloswine has Blizzard, which can erase Butterfree from existence in the blink of an eye - technically, some Butterfree might live a Blizzard, but their attacks do pitiable damage to Piloswine. it is not even worth risking the slim chance of a Sleep Powder miss; just use another Pokémon.
Petrel (Goldenrod Radio Tower): Butterfree can OHKO all of the Koffing with the Choice Specs and Psybeam. This does not hold for Weezing instead, which easily annihilates Butterfree with Explosion; find a better counter for it.
Rival (Goldenrod Underground): Much of this fight is very difficult, if not impossible, for Butterfree. Golbat can outspeed and hit with Air Cutter, Magnemite has Spark and will take little to no damage from Butterfree's moves, Haunter's Shadow Ball hurts and Psybeam is highly unlikely to OHKO, Sneasel's Faint Attack will leave an even greater dent than its Icy Wind, and both Quilava and Feraligatr are unapproachable because of their super effective moves. Of all the Pokémon that can be seen here, only Meganium is an opponent Butterfree would not be afraid of; it can also somewhat manage Sneasel by netting a Silver Wind 2HKO, but at the expense of risking a critical hit, as Faint Attack 3HKOs Butterfree. While Haunter is also theoretically defeatable, its Mean Look and Curse combo paired with Confuse Ray easily makes short work of Butterfree with just a little luck, and the risk is not worth the reward.
Proton (Goldenrod Radio Tower): Golbat's Wing Attack is a 2HKO like Butterfree's Psybeam, and Butterfree loses because it is slower. Weezing fortunately cannot explode, so Butterfree can resort to Sleep Powder and Psybeam to wear it down; its best move can only 4HKO Butterfree, so there is no risk.
Ariana (Goldenrod Radio Tower): Arbok is taken care of by the wonders of Sleep Powder and then two Psybeams. Vileplume does not represent a great danger, being capable of a 5HKO at most, but if Butterfree beats it, it will not be able to swap out scot-free against Murkrow, which has both Wing Attack and Pursuit. Unless Butterfree is playing Shift mode, Butterfree is probably better off backing away from the battlefield after defeating Arbok, or at worst switching out right after Murkrow comes only if its health is higher than half; this will allow Butterfree to take a non-critical Pursuit on the switch.
Archer (Goldenrod Radio Tower): Neither Houndour nor Houndoom are opponents worth risking Butterfree's life over. Houndour can be beaten with a combination of Sleep Powder and Silver Wind, or just 2HKOed by Silver Wind as well, but its Fire Fang will kill Butterfree if it crits. Koffing does not have bomb moves, instead, so a Psybeam with the Choice Specs (or two Psybeams without) will take care of it.
Gym #8 - Clair (Blackthorn City, Dragon-type): Nothing here can save Butterfree. Gyarados' Dragon Rage, the elemental attacks of the Dragonair, and Kingdra's moves - just about any of them - will all kill the butterfly before it can do anything significant, and probability makes it disadvantageous to bet on the Sleep Powder and switch, since the enemies will likely wake up too soon for the recipient Pokémon to set up. While Butterfree can theoretically take on one of the Dragonair with Silver Wind or Bug Buzz, if it can outspeed, there is no way of telling which one has Fire Blast as its last move and which one has Aqua Tail instead; their movesets are otherwise the same, and so are their levels and genders. Fire Blast can "only" 2HKO Butterfree, but Butterfree can only counterattack for more damage if it does have Bug Buzz and is holding the Choice Specs; without this item, Bug Buzz is not guaranteed to 2HKO Dragonair, putting the butterfly in danger. Overall, if it can be helped, do not use Butterfree here.
Kimono Girls (Ecruteak Dance Theater): Butterfree can win against Umbreon with Sleep Powder, Bug Buzz and some patience. Sleep Powder is not mandatory, but helps Butterfree reduce the damage intake from Umbreon's moves, hopefully nullifying it if the sleep turns last long enough (Bug Buzz is a 2-3HKO). Espeon is faster and 2HKOs with Psychic, so Butterfree can either hold the Choice Scarf and hope for no critical hits, hold the Choice Specs and improve damage output to bring Bug Buzz to OHKO range, or use Sleep Powder and cross fingers that the sleep turns last long enough for it to win. Flareon and Jolteon are nigh untouchable due to their strong moves, as both of their respective STABs can OHKO Butterfree. Vaporeon 2HKOs with Surf, so it is theoretically doable, but will off Butterfree easily in practice, both due to the damage taken from the other fights and because Sleep Powder will once again be necessary to have any hope whatsoever of winning. U-turning away from the battlefield is an acceptable other option, but with Butterfree's low Attack, U-turn will deal rather pitiable damage to anything not called Espeon. Given the disadvantage it has against most of the Eeveelutions, Butterfree is likely better off not leading at all.
Ho-Oh (Bell Tower, HeartGold only): There is no way Butterfree can take on a legendary with a STAB super effective attack. Stay out of this.
Lugia (Whirl Islands, SoulSilver only): There is no way Butterfree can take on a legendary with a STAB super effective attack. Stay out of this.
Rival (Victory Road): Most of the rival's team represents too big a threat for Butterfree. Sneasel can be OHKOed with Bug Buzz and Butterfree can take a Faint Attack from it; same for Kadabra, with Psybeam instead of Faint Attack. Meganium is "only" 2HKOed, but its Petal Dance deals laughable damage. These are the only Pokémon Butterfree should be fighting; Golbat, Magneton and the other two starters all have moves that hit Butterfree for super effective damage and can take multiple hits from Bug Buzz, while Haunter can Curse-trap Butterfree and confuse it with Confuse Ray.
Elite Four Will (Indigo Plateau, Psychic-type): A Butterfree holding the Choice Specs will OHKO all of Will's team with Bug Buzz, Xatu ace included. Given Butterfree's likely level advantage, it is improbable that Will's Pokémon will outspeed it; if that happens, Butterfree can take any one of their strongest hits at any rate: neither Jynx's Ice Punch nor Xatu's Aerial Ace can OHKO it.
Elite Four Koga (Indigo Plateau, Poison-type): Butterfree's Speed is its greatest asset in this fight, since none of Koga's Pokémon are particularly fast, Crobat excluded. Ariados has Insomnia, so it cannot be put to sleep; 2HKO it with Psybeam, instead. Venomoth can also be 2HKOed with Psybeam and deals relatively little damage with Psychic; Butterfree can attempt to Sleep Powder it and put up a Substitute to fight the rest of Koga's Pokémon better. Forretress and Muk must be fought only with a Substitute up; being slower than Butterfree, they can be put to sleep first, and Butterfree needs to keep a Substitute up against them at all times, since Muk's Gunk Shot KOs it after one Substitute is used (up to around 75% damage) and Forretress' Explosion is bloodshed regardless of circumstances. It should be noted that Muk has a Black Sludge, which makes Butterfree's Psybeam roughly a 4-5HKO alone, not counting Koga's healing items; it is very likely that, at some point, Butterfree will need to bail out, and if that happens, a good replacement that can take Gunk Shot well should enter the battlefield: prepare a backup. Butterfree also cannot win against Crobat whatsoever, as it outspeeds and 2HKOs with Wing Attack.
Elite Four Bruno (Indigo Plateau, Fighting-type): Hitmontop is Butterfree's best shot at anything remotely resembling setup material. With a moveset that only consists of two Fighting moves, a Ground move and Quick Attack, Butterfree can easily Sleep Powder it and put up a Substitute to try and keep throughout the fight, for as long as possible. After that, it can either Tailwind its way to victory by keeping on using Sleep Powder, or spam Psybeam until Hitmontop drops, and whatever comes in after it. Without an active Substitute, Butterfree cannot do anything else: Hitmonlee and Hitmonchan have Fire moves, while Onix and Machamp have Rock Slide. However, since Machamp is slower than Butterfree and has No Guard, Butterfree can actually hope to beat it if it manages to Substitute for long enough to keep the fight going, especially as its Sleep Powder will have perfect accuracy against it, thanks to Machamp's No Guard. Butterfree is highly unlikely to defeat more than two or three Pokémon, though: Psybeam can 2HKO only with luck, and will more likely 3HKO due to damage variation; Bruno's Pokémon, instead, 2HKO Butterfree with ease. How much contribution Butterfree will give to this fight depends on the whims of fate.
Elite Four Karen (Indigo Plateau, Dark-type): Bug Buzz works very well against Umbreon, 2HKOing it; Vileplume is manageable since its Acid does little damage. Butterfree might actually also beat Gengar with relative ease, but only with lots of strategic play: Gengar's Destiny Bond, paired with its Speed, can kill anything that cannot outspeed or paralyse it. Butterfree theoretically could, with Stun Spore, but this means forgoing the Substitute slot, which is rather useful against the rest of the Elite Four. Alternatively, Butterfree can stall out the Destiny Bond PPs and then start hitting with Psybeam, which is a 2-3HKO (2HKO is more likely); Gengar's best move is Focus Blast, which will barely tickle Butterfree. This is it: Murkrow destroys Butterfree with Sucker Punch and Pluck, and Houndoom can roast it in a heartbeat.
Champion Lance (Indigo Plateau, Flying-type): Everything on Lance's team has at least one super effective move to hit Butterfree with, and outspeeds it. Do not let Butterfree enter the battlefield.
From this point onwards, you can fight the gyms in any order, though you will need to retrieve the Machine Parts from the Cerulean City gym before you have access to the earlier portion of Kanto. Feel free to anticipate or postpone any battles as needed.
Gym #9 - Brock (Pewter City, Rock-type): Butterfree can only contribute to this fight if Erika has already been beaten, as it needs Giga Drain to function. This move will OHKO Graveler, Rhyhorn, Onix and, with Choice Specs support, also Omastar. It should be noted that Onix ties with Butterfree in Speed, so specimens with a Speed-lowering nature are better off avoiding Onix. Likewise, Kabutops is a near-certain outspeed, and can wreck Butterfree with Rock Slide.
Rival (Mt. Moon, optional): Butterfree can beat Meganium with Bug Buzz, and potentially leave a dent in Alakazam with the same move; Bug Buzz is an OHKO with the Choice Specs attached, but Butterfree needs to take a Psychic first, since Alakazam is faster. Psychic is a 2HKO, so a regular hit will not be a problem, but a critical hit will kill unless Butterfree is outrageously overlevelled. Sneasel can also be beaten, much like the previous fights, with Bug Buzz. Gengar is a 2HKO with Psychic, but it can also 2HKO with Shadow Ball, so only a Butterfree holding a Choice Scarf has any chances against it. The rest of the rival's team is too strong and too fast for the butterfly, so use a better Pokémon.
Gym #10 - Misty (Cerulean City, Water-type): Butterfree can 3HKO Golduck with Bug Buzz; Sleep Powder will help it take less damage from it before it goes down. Quagsire is also 3HKOed, and Butterfree's immunity to Earthquake makes it a good candidate for this fight; even better if Erika has already been beaten, which will give Butterfree access to Giga Drain. Sleep Powder is necessary against Lapras, and so is Substitute; its Ice Beam is very close to an OHKO and Butterfree will take at least three turns to beat it. Starmie, instead, can be fought only with a Choice Scarf or a Substitute already up: Ice Beam 2HKOs, and so does Bug Buzz, so Butterfree will need to either outspeed or get in a few sleep turns to prevail.
Gym #11 - Lt. Surge (Vermilion City, Electric-type): Considering that everything packs Shock Wave and is either as fast as Butterfree or significantly faster, this is not the place for Butterfree to be. Bug Buzz will 3HKO at best, and Lt. Surge's Pokémon can all do better than that.
Gym #12 - Erika (Celadon City, Grass-type): Butterfree takes very little damage from Erika's Pokémon except Tangela, which has AncientPower; however, Tangela is slow and OHKOed for certain by Bug Buzz, which makes it a non-issue. Jumpluff is 3HKOed by Bug Buzz, Bellossom is 2HKOed, and Victreebel is 2HKOed by Psychic. It might actually be beneficial for Butterfree to defeat this gym before any other, as it gives it Giga Drain access for the other Kanto fights.
Gym #13 - Janine (Fuchsia City, Poison-type): Crobat can be outsped with the Choice Scarf and several levels of advantage, and 2HKOed with Psychic; if any of these conditions do not hold true, however, Butterfree will lose to its Wing Attack. Weezing, instead, can be OHKOed by Psychic only if Butterfree holds the Choice Specs; this means Crobat and Weezing cannot be fought together. Psychic will also 2HKO both of the Ariados as well as Venomoth, all of which only 3HKO with their best move. Venomoth can be put to sleep if Butterfree is not holding a Choice item, but the Ariados have Insomnia.
Gym #14 - Sabrina (Saffron City, Psychic-type): Espeon is the biggest obstacle. It is faster than Butterfree, and can set up Calm Mind to quickly shrug off even Bug Buzz. Sleep Powder may look like a tempting choice for the first turn, but its Psychic will hit hard even at zero Special Attack boosts, and Butterfree will be in constant danger fighting it. The best option to take down Espeon is to give Butterfree the Choice Scarf; it will hit before Espeon can set up, and 2HKO with Bug Buzz with or without a Calm Mind turn on Espeon's end. Mr. Mime's Filter makes Bug Buzz only a 2HKO, which is still better than its Psychic, a narrow 3HKO; however, if Butterfree has fought Espeon before, it will need frequent healing against Mr. Mime, especially as the Choice Scarf makes Sleep Powder unavailable. Lastly, Alakazam can make short work of Butterfree despite the type disadvantage: even the Choice Scarf is not enough to guarantee outspeeding without a substantial EV investment, and Psychic 2HKOs just like Bug Buzz. Unless Butterfree is certain to outspeed and its health is still maxed out, do not fight Alakazam.
Gym #15 - Blaine (Seafoam Islands, Fire-type): Butterfree is ill-suited for this battle in each and every possible way, and should not even think of entering the field. While it could theoretically use Sleep Powder and Substitute to get past Magcargo, the risk is just not worth it, especially against a Pokémon that has so many other weaknesses, two of which are double.
Gym #16 - Blue (Viridian City): Bug Buzz is the perfect Exeggutor buster. A combination of Sleep Powder, Giga Drain and some luck (to not have Sleep Powder miss) may also help Butterfree defeat Rhydon, but a single hit from it will end the butterfly, and is not worth the risk unless no other options are available; Giga Drain is not an OHKO either, unless Butterfree is holding the Choice Specs. A Butterfree without a Choice item, instead, can try to beat Machamp: since it is faster and No Guard makes Sleep Powder perfectly accurate, Butterfree can alternate Sleep Powder, Substitute and Psychic until Machamp goes down; if Machamp stays asleep for enough turns (Psychic is a 3HKO), Butterfree will win. Everything else on Blue's team has super effective moves and stats that Butterfree simply cannot rival, so keep it away.
Rival (Indigo Plateau, optional): Once again, Butterfree's only easy targets are Meganium and Sneasel. Alakazam and Gengar can be beaten, but only if Butterfree holds the Choice Scarf; in that case, it can 2HKO them respectively with Bug Buzz and Psychic, but it will also get OHKOed by a critical Psychic or Shadow Ball from them, unless its level is significantly higher. Everything else on the rival's team should be fought by Pokémon better suited for the task.
Red (Mt. Silver): Pikachu, Lapras and Charizard all overwhelm Butterfree with their power; Lapras is slower and can theoretically be put to sleep, but Blizzard has an OHKO chance against Butterfree and the butterfly's moves barely do any damage to Lapras. Snorlax and Blastoise have Blizzard too, which makes Butterfree much better off avoiding them; in addition, Snorlax is a physical hitter and special tank by nature, and Giga Impact deals even more damage than Blizzard to Butterfree, killing it on the spot. Venusaur has Sludge Bomb but not much else; Butterfree should be able to beat it without problems, if it can keep it asleep for long enough. However, two can play at that game: Venusaur also has Sleep Powder. In the event that Butterfree is taking too much damage or getting poisoned too often (which makes Sludge Bomb a 2HKO, together with residual poison damage), prepare a backup.
Caterpie starts with Tackle and String Shot, and only learns Bug Bite if kept unevolved until level 15. Metapod learns Harden at level 7. This move is useful on Butterfree in some matchups, notably the one against Bugsy. Butterfree learns Confusion at level 10, a good move, and at level 12 it also gains simultaneous access to PoisonPowder, Stun Spore and Sleep Powder. As Butterfree will generally want two damaging moves and two powder moves, the recommended alternatives are Sleep Powder and Stun Spore: Sleep Powder takes care of slower foes for almost no damage, and Stun Spore can cripple Pokémon that are faster than Butterfree. At level 16, the first STAB move comes around: Gust. At level 18, Butterfree gets Supersonic, but it has relatively bad accuracy even with Compoundeyes, so Butterfree will generally prefer to keep Sleep Powder and Stun Spore. Ignore Whirlwind at level 22. At level 24, Confusion gets upgraded by Psybeam. At level 28, the long awaited Silver Wind becomes available; despite the typing, which does not do it many favours, it comes in handy against several foes. Tailwind, at level 30, is an okay support move, but Stun Spore is generally better. Safeguard, at level 34, is useless; so is Captivate at level 36. Lastly, at level 40, Butterfree gets Bug Buzz, its best move.
Unfortunately, almost all of the good options Butterfree gets by TM are postgame. Shadow Ball is the only exception. For those who nuzlocke Kanto, Giga Drain or Energy Ball are must-haves; Psychic is another good option, although Butterfree has an even better alternative, Dream Eater, which goes very well with Sleep Powder. Butterfree does get access to other moves as well, but its physical damage output is far too lacking to make good use of them.
If the Shadow Ball TM has already been used on another Pokémon, Butterfree may learn Ominous Wind for coverage instead, which is available in the Battle Frontier.
Ground-types: Even though Ground-types share the Ice weakness with Butterfree, and having shared weaknesses on a team can be dangerous, they are very necessary for Butterfree to function. Its glaring weakness to Rock can only be mitigated by a Rock resistance with super effective STAB, and Ground-types are better than Steel-types at covering this role, since they also help Butterfree against Fire- and Electric-types.
Rock-types: For the remaining Flying- and Ice-types, Rock-types are the best pick. Having a dual Rock/Ground Pokémon to cover all of Butterfree's weaknesses at once would be ideal, but running separate Pokémon is also possible, so long as an additional teammate covers for their shared Water weakness; Butterfree can take care of Grass-types for them.
Physical tanks: Butterfree's physical endurance is by no means great, and will fall short against any strong hitter with a regularly effective move. Aside from physical Fighting-types, Butterfree needs help against almost any other physical hitter, and a proper tank is necessary for this.
Minimum stats are calculated with 0 EVs, IVs of 0, and a hindering nature, if applicable.
Maximum stats are calculated with 252 EVs, IVs of 31, and a helpful nature, if applicable.
Caterpie Line Ratings
What Nature do I want? For Butterfree, a Special Attack or Speed boost will do best in the long run. Ideally, it should be Modest or Timid, so as to not dent its defensive stats further.
At what point in the game should I be evolved? Before fighting Falkner. Caterpie and Metapod are hardly ever useful.
How good is the Caterpie line in a Nuzlocke? Butterfree is a good Pokémon for a while, but it loses steam at about half of the Johto playthrough, and has serious trouble holding up until the end. While its Sleep Powder is extremely useful, its not so great Speed is often a problem to make proper use of it. Still, it is by far the best early game bug one can find in Johto.