He also dug up unused mega-evolution for Latias and Latios.
Pokemon X and Y has been out for a few weeks now, and if you thought you had "caught them all" you may want to think again. A hacker that goes by the name Smealum has been searching through the Pokemon X and Y source code and has discovered three, currently unobtainable, legendary Pokemon, as well as unused mega-evolutions for generation III legendaries Latios and Latias.
This is obviously a massive spoiler for those of you who would rather wait for Nintendo to officially reveal them, so i'm going to throw a spoiler tag around the gallery.
The Pokemon (and the mega-evolutions) will most likely be made available via special events, just like the mega-Blaziken event that we kicked off this generation with. The three Pokemon are namde "Diancie" (which looks like maybe a Fairy/Rock type), "Hoopa" (possibly Fairy/Ghost) and "Volcanion" (which, going by the name, is quite obviously a Fire type).
Update: A few sources are reporting that the typings are: Diancie - Fairy/Rock, Hoopa - Psychic/Ghost, Volcanion - Fire/Water