One of the characters in Pokemon X and Y remarked once near Laverre Town that they were once chased through the woods by what looked like a group of five faceless men. Now everyone look at vanilla Zygarde: The five brooding frills around its neck each have two glowing nodules, and if you were to create a silouhette of Zygarde looking at you in the darkness, it would look like five faceless men with glowing eyes.
Now look at the silouhette of this supposed new Pokemon. On its head is the same series of frills, albeit smaller and appearing more like a crown, but roughly the same shape and most importantly, the same number - five.
The one thing that seems to throw everyone off is the fact that this new form of Zygarde appears much more anthropomorphic. I can assure you that this is Zygarde, if not at least part Zygarde. Also look at the hexagonal glowing markings on its arms, which mirror that of the hexagonal glowing markings on Zygarde, as well as the hexagonal skin texture, AS WELL AS the animation for the moves Thousand Waves and Thousand Arrows.
Undoubtedly, this is Zygarde.