Xerneas may be based upon the Ceryneian Hind; a sacred Greek deer.
In Greek mythology, the Ceryneian Hind, also called Cerynitis, was an enormous hind (deer), who lived in Keryneia, Greece. It was sacred to Artemis, the chaste goddess of the hunt, animals and unmarried women. It had golden antlers like a stag and hooves of bronze or brass, and it was said that it could outrun an arrow in flight. The capture of the hind was one of the labors of Hercules.
If you look on Bulbapedia, it suggests that this may be the origin of Xerneas, along with Cernunnos (Celtic horned god)
The souls did from their bodies fly, - They fled to bliss or woe!