In the glint of its eyes is the shape of the letter "X", hence the title of the game and when it stands, its body seems to be in the shape of an "X".
Xerneas is the second Pokémon to have a name to start with X, the first one being Xatu.
Xerneas is the only legendary Pokémon which 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 that it legs are a similar form and Sawsbuck due to it looks like a Deer, has similar horn except Xerneas has multiple horns that are small, that are multicolored, and that branch off like a tree.
Xerneas resembles the Forest Spirit from Hayao Miyazaki's film, Princess Mononoke.
Xerneas is the first legendary Fairy-type Pokémon.
Xerneus 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.