As most of us expected, it's getting two alternate forms, labeled 10% and Perfect. Perfect Zygarde is the silhouette we've seen before, and it's said to exceed Xerneas and Yveltal in power... though Kyurem had the same thing going for it, and we all know how THAT went. It's safe to assume Perfect Zygarde will have 700 BST - and it's also possible fusion of some kind is involved, since it has some blue and red on it. As for the 10% form (the one that looks like a dog), we don't know much about it, except that it's really speedy. We also don't know whatever its stat total could potentially look like, and there's still no word on how Thousand Arrows and Thousand Waves will work. Finally, the tiny blobs we've seen are the Zygarde Core and the Zygarde Cell, which makes up the mind of the new forms. No idea how it's going to translate in gameplay terms... we still have a lot to learn about all these new things, such as how the transformations are triggered in the first place, although we at least have the basics figured out.
There's also supposed to be some palette-swapped Greninja or something, but there's no word on what it's supposed to do. I'm just hoping it's not a further buff or anything, because it does NOT need it.