The Internet has been buzzing about this potentially new Mewtwo Forme. The picture was apparently leaked earlier in the week and is just now making it to major Pokemon sites like Serebii and Pokebeach.
Personally, it doesn't seem real to me. The main thing that bothers me is the tail that now is growing out of the back of Mewtwo's head. It also seems odd how we would be potentially be getting this new Forme in Pokemon X and Y. Normally, formes are reserved for games that come out later in a generation because Nintendo can't add new Pokemon within a single gen, but they can add formes. For example, Rotom and Giratina formes in Platinum and most recently the Kyurem Formes and Therian formes in Black 2/White 2.
Does that make it impossible that this Forme is real? No... but it would seem unlikely. Personally, if it is real, I'll keep my Mewtwo in its regular Forme. There is going to be an event Mewtwo released with the next movie, so if it does have something to with X and Y... either a new Forme or ingame event... I'm sure we'll know soon.
*UPDATE: This has been confirmed to be a new Mega evolution: MegaMewtwo. See Pokemon.com for details!