My guess is that it is either:
A: A new clone of Mew. Someone tried to reattempt/improve Giovani's experience, giving us a new itteration of Mewtwo.
or
B: A clone of Mewtwo.

I'm possitive that this guy is NOT Mewtwo. I'm sure Mewtow doesn't evolve or ''change'' into this guy. Why you ask? Because the opportunity of a pokémon movie with Mewtwo facing this guy as his new nemesis is to good to ignore.

 

What?! I can't hear you over all this awsome! - Pyrrhon (Kid Icarus:Uprising)

Final Ultimate Legendary Earth Power Super Max Justice Future Miracle Dream Beautiful Galaxy Big Bang Little Bang Sunrise Starlight Infinite Fabulous Totally Final Wonderful Arrow...FIRE! - Wonder-Red (The Wonderful101)