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Xerneas is a blue and black, quadruped, deer-like Pokémon with an eight-horned set of antlers extending out from its 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 its head, and the root of its horns mark its face, just below the eye and over the nose. Its eyes are the same color as its fur, and its pupils are X-shaped. The blue portion of its body ends at the chest area where the "fur" jousts outward, similar to the blue horns on its face.
The rest of its body is black with small, multicolored spots on its back (the same color and order as its horn adornments, the teal-blue colored-spot on the base of its tail) and a blue stripe running along its spine. It has a small, fluffy black tail where the blue stripe ends. Its 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 its forelegs further map a crude sword-like pattern.