CoroCoro have confirmed that it is indeed a new forme of Mewtwo, not a new pokemon. Although this is Mewtwo having "awakened" if the next movie title is anything to go by, so hopefully that means it will be all the more awesome, because Mewtwo is already one of the most dangerous pokes in the game (Does anyone here play on Pokemon Showdown/Pokemon Online, by the way?). Game Freak are trending towards introducing increasingly monolithic stat lines á la the Kyurem formes, so it's likely.