Go wanted to catch as many Pokémon as he could and one day while in a forest in the Kanto region, he caught every Bug type Pokémon native to Kanto. It was spotted on the top of a tall tree which Go climbed in order to get Butterfree.
Butterfree resides at a habitat at the Sakuragi Institute in Vermilion City. It is not the evolved form of
Goh's Caterpie or
Goh's Metapod as they are separate Pokémon.