Butterfree made a cameo appearance in Ash Ketchum vs Charles Darwin.
Information on the cameo
Butterfree is a dual Bug/Flying-type Pokémon based on the real-life insect known as the butterfly. Its species is the Butterfly Pokémon, and it evolves from Metapod, which evolves from Caterpie.
In the anime, Butterfree was the first Pokémon that Ash Ketchum caught, as well as the first one he evolved, first catching it as a Caterpie before evolving it into a Metapod, then a Butterfree. He released his Butterfree in the anime episode "Bye-Bye Butterfree", additionally marking it as the first Pokémon that he would release.
Appearance in the rap battle
Butterfree flies across the scene in Ash Ketchum's first verse during the line, "You got candy raps: Reese's Pieces!"