It is not known at this point how Diancie will be incorporated into Pokémon X and Y, but you can be sure it will be in some future version. This also makes it officially the 719th Pokémon (as if more were needed).
Diancie was leaked back in November with its Mega Evolution shown on screen. So there is a chance it will come in the form of DLC.
Pokémon XY: Cocoon of Destruction and Diancie is set to hit Japanese theatres on July 19th. Remember when you were at the first Pokémon movie way back in 1998? Send us some of your Pokémon memories in the comment section below. For more info on Diancie, check here. For our review of Pokémon X and Y, click here.