Ah; who runs the Dragon Nation...really there are so many options.
We can automatically rule out... Giratina, Dialga, Black/White Kyurem, and Zekrom, since they rule other nations, as well as Arceus since he rules the whole region... that leaves the more-logical answers to be either Latias/Latios, Trevor, Palkia, Reshiram, or Zygarde....
So I'm gonna guess Zygarde.