Pokemon Mystery Dungeon: Blue Rescue Team

Pokémon Mystery Dungeon opens with the player (formerly a human) waking up as a Pokémon. The Pokémon that woke him up explains that he found the player just like that; unconscious on a field. The player quickly comes to the realization that he has been turned into a Pokémon, but apparently has amnesia (except for the memory that he was previously a human). The partner explains that the Pokémon world has recently been riddled with natural disasters (these disasters are also preventing Pokémon from evolving). Just then, a Butterfree flies up, and in a frenzy asks the two to help her find her lost child, a Caterpie. After saving the child, the partner and player Pokémon decide to form a Rescue Team.

Source: TheGamesDB