Remember that rather odd picture of Mewtwo that surfaced a few months back? Well, now we know what the hell that was: a MEGA-EVOLUTION!
UPDATE: We've got a new trailer for you.
Apparently, according to CoroCoro (translated by Serebii via Polygon), there'll be a whole new level of evolutions called Mega Evolutions in Pokemon X and Y. Only a select bunch of Pokemon will be able to Mega Evolve, such as MegaBlaziken, MegaAbsol, MegaMawile, MegaMewtwo, MegaLucario and MegaAmpharos. There'll be a nice new gym trainer who'll explain all of this and help players out with Mega Evolutions via a process involving a "Mega Stone."
This is probably the best thing I've heard all week. You can find a bunch more information and Japanese magazine translations here.
Pokemon X and Y are due to release this October.