Xerneas is a quadrupedal stag-like Pokémon with a blue and black coat. Along its back it has multicolored spots that glow when it is in battle. It has long, gleaming yellow antlers that shine with all of the colors of the spectrum; its antlers resemble the roots of a tree. Its yellow-lined hooves are pointed, and it has a long blue and black tail. When Xerneas is not active, its multicolored antlers stop glowing and turn cyan blue.
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.
As of Gen VII there is also the Ultra Beast Xurkitree.
Xerneas is the only legendary Pokémon whose name starts with X.
Xerneas bears a resemblance to the Legendary Pokémon 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 X-shaped pupils are rather ironic as in many cartoons crossed-out eyes indicate death.
Xerneas moves along the ground. This could be a reference to the x axis on a graph, being horizontal.
Similar to Kyogre, Xerneas' moveset has an advantage over all other members of its trio. Fairy attacks on Yveltal and Zygarde are super effective.
Xerneas may be inspired by a deer, most notably the stag that grazes upon Yggdrasil or perhaps the Cernian Hind of Greek Mythology. It also shares apects of the Tree of Life.
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. It is referred to be part of Yggdrasil Trio.
Xerneas as it appears in Super Smash Bros. for 3DS / WiiU.