A new pokemon has been confirmed for Pokemon X & Y, the 6th generation of Pokemon games which will be released on the 3DS.
This pokemon remains without an official name at the time of writing, but if you look at the screenshot below you can clearly see a strong link to the legendary pokemon Mewtwo. (You can click for full size)
It is unclear whether this going to be a pre Mewtwo or post Mewtwo evolution, but it is pretty clearly a direct relation to Mewtwo. In terms of Pokemon lore and canon, a post evolution would make the most sense as Mewtwo himself is a clone of the legendary pokemon Mew. Another possibility is that this pokemon was created by Mewtwo, or is his offspring. That would also make more sense than a pre Mewtwo evolution.
This unnamed pokemon is joining the ranks alongside Ninfia (a Light type evolution of Eeevee), the starters Chespin, Fennekin and Froakie and the legendaries Xerneas and Yveltal.