SPOILERS! Update: Chart of Mostly All New Pokemon in X/Y Leak Thread (Enter at Your Own Risk!)

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Also there is a theory that the legendaries are rooted in Nordic mythology, and Zygarde being serpent-like pretty much confirms this.
http://www.zeldainformer.com/news/pokemon-x-and-y-based-on-norse-mythology-third-legendary-will-be-a-serpent