Board Thread:Pokemon Discussion/@comment-5398529-20131027020555

In Pokemon Y, the entry states what it does: "monitor the ecosystem".

However, in X, the entry is much different, going as far as to hint what it might do in Z.

"When the Kalos region's ecosystem falls into disarray, it appears and reveals its secret power."

Most likely in Z, Team Flare will try to use Yveltal and Xerneas to wreck havoc and then recreate the world in their own image. Zygarde will sense the disturbance, and use its "secret power", probably an alternate form with higher BST, to stop them.

What do you think? 