Okay, now that I've been linked to that other image, I can at least see it in the screenshot. I can't see it on the page, but maybe it uses some extension I don't have on this computer (and am not going to put on it.)
Do you use Firefox? Not sure about other browsers, but you can right click, use "inspect element" and delete portions of the page (they'll come back after a refresh, no worries)
I would think (for balance reasons): Xerneas - Fairy/Grass Yveltal - Dark/Flying (thus both legendaries are strong and weak to each other, as I don't think they'd want one legendary to have an advantage over the other)
and perhaps the third Legendary (Z?), which has been speculated to be a snake, could be Poison/some other type strong against Yveltal so that it would have an advantage over both of them.