I am not going to put forward my ideas on what it is, but I will point out Nintendo having a history for leaking pokemon ahead of game releases in thier movies: Recall, if you will, the FIRST movie (as my example, it's perfect), where the mini-movie starred Marill, and the feature itself showed a Donphan, long before the games themselves were released. Note the fact that they had absolutely nothing to do with the actual storyline of Mewtwo's cloning etc, and yet were leaked in that movie anyway.
Thinking along this line, it's easy to assume that this new pokemon could be an evolution of, say, Mightyena, or a new pokemon, etc, without it needing to have anything to do with the actual storyline (much like WPM mentioned, this pokemon was not specifically marked as the "Z"pokemon).
In fact, we may see more new pokemon in this movie than this "Z"pokemon, this mystery pokemon we are currently ga-ga over, or any new forms (as it was speculated that Celebi might get a new form in this movie from some new merchandise marked as celebi (?)... Yet to have that confirmed as far as I know.)
As for me, I hope this movie leaks several of the new pokemon, but leaves enough mystery to keep the games interesting (I mean, what fun is it knowing every pokemon in the next gen before the games are even released?)
Also, can't wait for more coro-coro scans and next week's pokemon sunday, since leaks dont have to be confined to this one pokemon, now do they?
, I hope it's the evolution of lucario because it's one of my favourite pokemon
