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 made its first anime appearance in M17 along with Yveltal and Diancie.
A Xerneas appeared in XY040: Foggy Pokémon Orienteering! where it was first seen by Bonnie and Pikachu. After they were saved by the remaining Team Froakie members and Team Squirtle, the two groups saw it again before it retreated to the forest shadows.
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.