Mightyena will probably be throughout most of the movie and evolve at the end into a Pokemon that helps Ash defeat Z and whoever is using it. That's my theory, at least, if it is indeed a Mightyena evolution. I mean, Z is probably a legendary of some sort, and I honestly doubt they'd reveal a legendary first. Just doesn't seem right. Z will be revealed soon, I'm guessing, but I doubt it's this Pokemon.
If it's not a Mightyena evolution? Eh, we'll see. I doubt this is Z, however, it seems much more likely that this is a "mascot" for this gen, such as how Munchlax and Lucario (particularly Lucario) turned out to be for 4th gen when all was said and done, regardless of whether or not it's an evo.
EDIT: I think it's the Pokemon that defeats Z because it's the "key to Z". I see that as being the key to the mystery of Z instead of being Z itself. We'll see in a couple weeks. Could be Z itself, though, we really have no idea, though it'll be good no matter what it is. However, I'm just saying, don't rule out the possibility of it being an evolution, especially since it's the first revealed Pokemon.