The next entry of the japanese magazine CoroCoro has leaked and as Expected many things were explaind and announced.
Apparently Zygarde has 2 new forms, a dog-like form called "10% Form" and the one showcased in the trailer of the movie Pokémon XY 2015, which is now called "Perfet Form" . The Zygarde we are use to is now called "Zygarde 50% form". Zygarde is said to mantain it's typing in this different forms but it's height and weight will drastically change, the Perfect Form is also said to be stronger than both Xerneas and Yveltal. It was also revealed that the anime will be called "Pokémon XY & Z" as predicted by many people. It was shown a mysterious Greninja who apparently looks like Ash, but since it's appearance looks too similar to Greninja to be a MegaEvolution it seems likely that Ash will fuse with his new starter, as he did in the past with Pikachu.
It might also mean that to defeat Zygarde/Lysandre, the protagonist will need to fuse with his main Pokémon in the next Pokémon game.
The Pokémon Z announcement will probably come in the next few days so stay tuned !