Another interesting thing based on a post i saw on tumblr, but as it has been pointed out, X and Y not only relate to DNA, but also to the dimensions of space, along with Z. While the Great Forest Spirit is obviously land-based (width) and the flying legendary is sky-based (height), the 3rd legendary in the trio could possibly be some interpretation of depth, or of some alternate dimension. (Time isn't a dimension, but i'll let it slide for now, if that's how you wanna view it.) My best guess would probably be depth as in underwater, also forming a nice land + sea + sky trio.