Yesterday scans appeared online from Japanese magazine CoroCoro showing 'mega' Pokemon evolutions and today Nintendo has officially confirmed the changes with a new trailer.
Mewtwo, Lucario, Blaziken are among the Pokemon who have received mega evolutions for Pokemon X and Pokemon Y. Pokemon do evolve naturally to their maga forms and instead you need a mega stone which acts similarly to a sun stone and other such evolutionary items in past games.
Interestingly, mega evolution is only possible during battle and it's not clear if Pokemon revert to their normal state after the battle ends.
It's also unclear if every Pokemon has a mega form.
Pokemon X and Pokemon Y are set for worldwide release on October 12th and you check out the pre-order incentives for the games here.