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 is the first legendary Pokémon to have more than 2 horns.
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.
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.