Xerneas is life, Yveltal is Destruction(which could mean death) so the Z Pokemon will be what? Rebirth or something like that?
If so the only "animal" that comes to my mind right now is the Phoenix, but since Yveltal already resembles a "bird" shaped Pokemon I don't think that will be the 3rd legendary...so following the form of the other 2 legendaries which have X or Y elements on their design I will say a Snake,Dragon,Basilisk is most likely to be the Rebirth Pokemon.
Anyway what are your thoughts about the Z legendary?
Oh another one...what if the Z legendary is based on immortality? (yeah i took the idea from the "leaked" kalos dex) it could have the infinite symbol( Something like this http://media-cdn.tripadvisor.com/media/photo-s/01/42/2e/f4/my-new-tattoo-ouroboros.jpg) and of course still be a Basilisk, Snake, Dragon, Ouroboros like being