Mimikyu

Upon summoning, Mimikyu acquires all normal characteristics (except color) of any one creature in play on either side; any enchantments on the original creature are not copied. During controller’s upkeep, Mimikyu may take on the characteristics of a different creature in play instead. Mimikyu may continue to copy a creature even after that creature leaves play, but if it switches it won’t be able to switch back.

Its actual appearance is unknown.