Xerneas are blue and black, quadruped, deer-like Pokémon with an eight-horned set of antlers extending out from their head, two on each side. Each horn is beige and adorned with a colorful, three-part protrusion twining through the horn (orange, red, purple, and teal-blue), one side mirroring the other. The colors gradually turn lighter to darker when moving from base to tip. Xerneas's head is blue with two horns on either side of their head, and the root of their horns mark their face, just below the eye and over the nose. Their eyes are the same color as their fur, and their pupils are X-shaped. The blue portion of their body ends at the chest area where the "fur" jousts outward, similar to the blue horns on its face.
The rest of their body is black with small, multicolored spots on their back (the same color and order as their horn adornments, the teal-blue colored-spot on the based of their tail) and a blue stripe running along their spine. They have a small, fluffy black tail where the blue stripe ends. Their legs are fashioned like blades with forward-pointing "hooks" at the tips, creating a cross shape, with the main "edges" trimmed with the same beige as the horns; the trim on their forelegs further map a crude sword-like pattern.