In the Pokémon TV series, Ash releases Butterfree after it falls in love with a pink Butterfree.
The story occurs in “Bye Bye Butterfree,” an early episode in which Ash’s Butterfree encounters a rare pink Butterfree during migration. The two Pokémon form a bond, but the pink Butterfree is initially rejected by the swarm because of its unusual coloration. Ash and his friends help protect it from Team Rocket.
After the pink Butterfree is accepted by the swarm, Ash chooses to release his own Butterfree so the pair can remain together. The decision is one of the series’ earliest examples of Ash placing a Pokémon’s happiness above his desire to keep it on his team.
Ash does not release Caterpie directly; it evolves first into Metapod and then Butterfree. This distinction is important when identifying the Pokémon involved.