Well even if there is not supposed to be any norse connection it fits pretty well into the yggdrasil myth.
Yveltal as Hraesvalg the bird at the top of the tree
Xerneas as Eikþyrnir the stag that lives in it's branches (think the one stag became four stags in later versions though)
Zygarde as Nidhuggr the dragon that gnaws at the roots of the tree (meaning the ground type is really fitting)
So I at least have my nicknames for them XD