For those saying it is not a new form but an entirely new pokemon, CoroCoro says otherwise.
From Serebii:
[quote][color=#FFFFFF][font=Verdana][size=1][background=rgb(56, 56, 56)][size=4]Though it doesn't reveal anything new, CoroCoro also reveals the pieces about the new Pokémon revealed last week. In doing so, it confirms that it replaces Mewtwo on the movie poster and confirms that it is in fact a new form of Mewtwo, with the poster explanation stating that Mewtwo has awakened and become its new shape. Thanks to gin for translations. We'll provide more on this as and when it comes[/size][/quote][/background][/size][/font][/color]