From Bulbapedia, the community-driven Pokémon encyclopedia.
Wayfarer Island (Japanese: ワタリ島 Watari Island) is the eighth island Ash, Iris and Cilan visited during their travels through the Decolore Islands.
This island is known for swarms of Caterpie, which, in order to migrate, evolve into Metapod and then Butterfree. Them doing so reminded Ash about the time he had a Butterfree. While all other Caterpie quickly evolved, one Caterpie was left behind, and Ash helped it to evolve all the way into Butterfree. Ash began his training with Caterpie and sent out Leavanny to wake it up every time it fell asleep.
When Ash was getting some Oran Berries, an Ursaring arrived and chased Ash to a cliff. However, Caterpie emitted a stench to prevent to Ursaring from doing anything else but it only made Ursaring more angry. It tackled Ursaring in the process and then evolved into Metapod. It still retained its lazy personality until Team Rocket arrived and stole it along with Pikachu. Metapod evolved into Butterfree and defeated the trio. With the task finally accomplished, Butterfree flew off with Ash sadly saying farewell to it.
Pokémon seen on Wayfarer Island