If you've been following news for Pokemon X/Y, you'll know that Gamefreak is introducing us to a new version of the legendary Pokemon Mewtwo. It has not officially been announced what this alternate version is, but the consensus seems to be that this is not an evolution but a "form," as seen in other Pokemon like Deoxys.
This new form of Mewtwo will be showcased in the upcoming Pokemon movie, ExtremeSpeed Genesect: Mewtwo Awakens. In this trailer for the movie, we see Mewtwo change into his new form. The movie is set to release in Japan on July 13, 2013. Look for the new Pokemon games on October 12, 2013.