Frieza Mewtwo confirmed
[Update: it looks like that scan revealing the new Mewtwo for the upcoming Pokemon film was correct!]
Today the Japanese variety show Pokemon Smash revealed a new form of Mewtwo for Pokemon X and Y, that essentially looks straight out of Dragon Ball Z. In-game footage was also shown, which makes him look significantly cooler than he seems at first glance. You know, since he has a giant tail thing coming out of his head.
Lots of theories are going around as to how this Mewtwo is obtained. One of my Pokemon enthuiast friends noted the similarities to Mew's simplistic design, and given the fusion-esque forms of Kyurem, anything is possible, including a theory involving the DNA Splicer.
Even then, the official YouTube account for Pokemon notes that he's "a New Pokémon with a Familiar Look" -- which may either be a misnomer, or a hint that it's an all new 'Dex entry.
So here's a recap of Pokemon X and Y so far: