Hoopa

Hoopa is a dual-type Psychic/Ghost Mythical Pokémon introduced in Generation VI. While it is not known to evolve into or from any other Pokémon, it has an alternate form, which it changes into by using the Prison Bottle. Its default form, Hoopa Confined, will then become the Psychic/Dark Hoopa Unbound. Hoopa Unbound in the party will revert into Hoopa Confined if three days have passed since changing to Hoopa Unbound. Other circumstances may also cause it to revert. (Source: Bulbapedia)