Pokémon Mystery Dungeon: Team Go-Getters out of the Gate!(2006)

Overview:

Charmander's voice calls out from the darkness if Squirtle is okay. The sleeping boy wakes up, and can't believe himself when he sees a talking Charmander. Charmander then points out that he's a Pokémon too. The boy takes a look at his reflection at a river besides him, and sees that it is true! He has turned into a Squirtle, and has no recollection at all how this had happened.