The Mythical Pokémon Hoopa's Unbound form has been revealed in a video and on the official Pokémon website today. As the video shows, Hoopa Unbound becomes a Dark/Psychic-type Pokémon; the official Pokémon website also lists it as being over 21 foot tall and weighing over 1,000 lbs.! That's quite a remarkable transformation for Hoopa to undergo.
Hoopa Unbound is available only in Pokémon Omega Ruby and Pokémon Alpha Sapphire. Details on how to unlock Hoopa's Unbound form, as well as when Hoopa itself will be available, have not yet been revealed.