If you meant in Fire Red :
While officially the "Mouse Pokémon," Pikachu’s design is heavily inspired by squirrels, particularly its cheek pouches and tail shape.
Are you trying to complete your Pokédex or just looking for some advice on how to catch these cute squirrel-like Pokémon?
Based on rats, but often lumped into the "vermin/rodent" category alongside squirrels in the Pokémon world.