According to Nintendo - "The mysteries of Pokémon X and Pokémon Y continue to grow with the unveiling of a Pokémon that is strangely familiar! It looks a lot like the powerful Legendary Pokémon Mewtwo, but not quite the same…"
So apparently this newly discovered Pokémon is somehow connected to Mewtwo, but it isn't strictly a new form. I'm betting in X and Y we find out the connection. Could it be Mewthree? This means every friend that told you their dad's friend's brother worked for Nintendo was RIGHT!
Just kidding, but maybe Mewtwo was messing around with cloning itself to create a child type thing... who knows?