Xerneas is the second Pokémon to have a name to start with X, the first one being Xatu.
Xerneas is the only legendary Pokémon whose name starts with X.
Xerneas bears a resemblance to the Legendary Pokemon Yveltal due to the fact that its legs are of a similar form, and there is also a resemblance to Sawsbuck since it of course is deer-like; it has similar antlers, except Xerneas has multiple points, some of which are small, some multicolored, and some that branch off like a tree. Sawsbuck, underneath its seasonal flares, only has two rather low-key racks of antlers.
Xerneas resembles the Forest Spirit from Hayao Miyazaki's film Princess Mononoke.
Xerneas is the first legendary Fairy-type Pokémon.
Xerneas represents life and Yveltal represents destruction.
Xerneas moves along the ground. This could be a reference to the x axis on a graph, being horizontal.
Its name may be reference to the Celtic "Horned god", Cernunnos. Its form may be a reference to Norse mythology guardians of the Yggdrasil tree: a bird-like creature that protects the top of the tree, this may be Yveltal, a deer-like creature that protects the soil and trunk of the tree, this may be Xerneas, and a snake-like creature that protects the underworld below the tree is Zygarde.
Xerneas as it appears in Super Smash Bros. for 3DS / WiiU.