Though we're rather close to the official release to Pokemon X & Y, there are still new snippets of information that are bubbling up. Today, for example, CoroCoro magazine scans that showcase three new Mega Pokemon appeared on Serebii.
Obviously, if you don't want these Pokemon spoiled, you should probably stop reading now!
Otherwise, you can find the scans of Mega Gengar, Mega Aggron, and Mega Tyranitar here. Another Pokemon, Mega Kangaskhan, also appears—but it had been confirmed earlier this year, along with other Mega Pokemon.
All the other Pokemon have a number of images floating around online purporting to be from early copies of the game—and they seem to confirm the CoroCoro scans.
A video by TheOneIntegral showcasing a couple of different screenshots:
Again, it should be noted that the last two screenshots are of the balloon Pokemon you play with in a minigame in X & Y and NOT of the actual Pokemon. However, their existence seems to corroborate the CoroCoro scans in one way.
Pokemon X & Y officially releases on October 12th, though some copies of the game seem to have leaked already. And in case you needed a refresher: Mega Evolution refers to a temporary power-up that Pokemon can gain in battle. In order to perform it, players need to find the appropriate Mega stone in Pokemon X & Y. You can learn more details about Mega Evolutions here.