Right, let's get this sorted out a bit here. There seems to be some confusion going around. I'll just lay down some facts and we'll go from there
Fact the first: This Pokémon REPLACES standard Mewtwo on the poster for the movie Extremespeed Genesect: Mewtwo Awakens. This indicates that it is likely to be Mewtwo. This is further confirmed by the official Japanese site promising the new trailer on April 18th featuring Mewtwo and Newtwo, where it says secrets shall be revealed
The official sites say that it's a new Pokémon. This has occurred before, most recently with the reveal of Black & White Kyurem where Smash, and Junichi Masuda, labelled them as new Pokémon. This become a point for discussion for a while until we managed to get the artwork from press sites seen to be labelled as 646_black and 646_white.
This to me at least indicates that it's a new form. However, who knows, it could be some new variation introduced in XY
It's all moot anyway, we'll find out next week :p