The pink Butterfree is the first alternate colored Pokémon in the anime. It was created before alternate colored Pokémon appeared in the games, possibly to differentiate it from the other Butterfree in the episode. This may be why alternate colored Butterfree are not pink in the games. Its voice actress in both Japanese and English is Chinami Nishimura
It appeared in Bye-Bye Butterfree. Ash's Butterfree fell in love with it, and fought to save it from Team Rocket. At the end of the episode, the two became mates and Ash's Butterfree flew away across the ocean with the pink Butterfree.
The only other time the pink Butterfree was seen again was in the Japanese opening Spurt!