MegaPikachu, use MegaThunderbolt!
Pokémon X and Y will support a new type of evolution called Mega Evolution, as shown in a recent issue of CoroCoro.
While the details of performing a Mega Evolution have not been fully disclosed yet, a few new forms have been shown including: MegaBlaziken, MegaMewtwo, MegaAbsol, MegaMawile and MegaLucario.
Mega Evolution gives the Pokémon new abilities (such as giving MegaMewtwo Insomnia), and alters their physical appearance.
We're still waiting on a full translation of CoroCoro for all the details, but it appears the new version of Mewtwo that we've seen in the past is, in fact, MegaMewtwo.