Yveltal is literally a Y, so you would assume that there’s an X on Xerneas. But I don’t see it. Does anyone else on the vast internet see it?
Look at it's horns. If you look at them from above, they'd likely look like an X.
Also, on its chest, there are 2 sharp points, and if you look at it and it's legs together, it looks a bit like an X.