From Bulbapedia, the community-driven Pokémon encyclopedia.
Lake Slowpoke (Japanese: ヤドン湖 Yadon Lake) is an anime-exclusive location found in between Mahogany Town and Blackthorn City. It appeared in Enlighten Up!. It is more commonly known as Lake Enlightenment.
Ash, Misty, and Brock arrived here when they were suddenly confronted by Team Rocket. They started a battle but were interrupted by a Slowpoke, which was revealed to belong to Madeleine, a student of the Slowpoke Temple, close to Lake Slowpoke.
After visiting the temple, Ash, Brock, Misty, and Madeleine went back to the lake were Misty wanted to fish. When Team Rocket stole the golden Slowpoke statue of the Slowpoke Temple and came rushing with it towards the lake, Madeleine's Slowpoke evolved into Slowbro after catching a Shellder with its tail. Slowbro managed to return the statue and defeat Team Rocket.
Pokémon seen in and around Lake Slowpoke