While Game Freak initially tried keeping their future event Pokemon under wraps, it appears we will finally be officially introduced to Diancie, who will be featured in the upcoming movie Cocoon of Destruction & Diancie.
Diancie had initially leaked months ago, along with two other event Pokemon. Until now however, Game Freak did not acknowledge any of these Pokemon officially. This Diancie announcement came in the Japanese magazine CoroCoro.
The Pokemon is a Rock/Fairy type and is #719 in the PokeDex. It is known as the Jewel Pokemon, obviously because of the prominent appearence of jewels on its body.
Nothing is known yet on when the Pokemon will be distributed for fans to download to their copies of Pokemon X and Pokemon Y, but it is safe to assume it will coincide with the release of the movie.
This now leaves Hoopa, the Psychic/Ghost type, and Volcanion, the Fire/Water type, as the only two remaining Pokemon who have yet to be officially revealed.