Ditto

Ability

Transform
During your turn (before your attack), you may put a Basic Pokémon from your hand on top of this Pokémon. (This does not count as playing that Pokémon or evolving.) This Pokémon is now that Pokémon. (Any cards attached to this Pokémon, damage counters, Special Conditions, turns in play, and any other effects remain on the new Pokémon.)

It can reconstitute its entire cellular structure to change into what it sees, but it returns to normal when it relaxes.