I was watch the Pokemon X and Y trailer and I noticed that Xerneas seems to have an X in its eye at a few seconds before 1:25. Would that mean it's the mascot of Pokemon X or could I have been seeing things?
Yes I think this is a right prediction as the other bird legendary is in the shape of a Y when you look closely. Also the tips of the "X" in the logo have the antler design and the tips of "Y" have a bird feather design.
So: Gazelle = X Bird = Y