I hope that is a new pokemon. MewThree for instance. I don't want it to be a new form because there are already so many different forms of legendaries: The genies, Black Kyurem, White Kyurem, Meloetta and genesect. If it is a new pokemon, it will probably be a post-game character (maybe event only).
It looks quite ugly. I think they messed up with white kyurem because it was so overdesigned.