Thanks to Kotaku we have known that the hacking community has discovered three new Pokemon (not currently available in Pokemon X & Y) looking into the game’s code.
Diancie, Hoopa and Volcanion appear to be “event Pokemon” which means players can only catch them at official Pokemon events.
Smealum, the hacker who has claimed responsibility for the hack, has also revealed Mega Evolutions for Latios and Latias. The hacker has also said that while it would be fun “in theory,” he didn’t want to “ruin” the online experience and he does not plan to release the hack or any of the discovered Pokemon.