Although it has not been officially confirmed, many people believe that this is the origin of the legendary trio. It is about the Yggdrasil tree of Norse Mythology. It's very interesting, and the theory is supported quite well.
Also, the first comment on that page, even though it's not that supported, is:
>have you seen the background of xernas it has a tree and pretty sketchy hmm but I think it also may be on greek myth apollo and artimis. apollo is a guy and guys have the y chromozone and his creature is a fiery wayvern. artimis is a girl so she has x and her creature is a deer but I don't think it's a chromozone cuz ppl might find it sexist and the fact it'll be weird. the serpent part is VERY likely cuz what other think could look like a z and if this theroy is true then it might be dragon,dark or ground.
According to Bulbapedia's page on Yveltal, Yveltal was supposed to in fact be based on a wyvern. Interesting, is it not? ;)
Hope I helped. :)
Source: Above Link