Xerneas is a blue and black, quadrupedal, stag-like cervine Pokémon with an eight-horned set of antlers extending out from its head, four on each side. Xerneas's coloration changes slightly depending on what mode it is in: Active Mode or Neutral Mode. In Active Mode, each horn is pale gold and adorned with a colorful, three-part glowing 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. In Neutral Mode, the horns are pale blue and the protrusions the same blue as its upper body. 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" juts outward to the side, similar to the blue horns on its face. On its breast is an inverted V-shape that is also pale gold in Active Mode and pale blue in Neutral Mode. The inside of the V-shape is black and extends into its lower body.
The rest of its body is black with a blue stripe running along its spine; when it is in Active Mode, the shine from its horns decorate its back in small, multicolored spots. It has a slim, 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 color as the horns (pale gold in Active Mode, pale blue in Neutral Mode); the trim on its forelegs further map a crude sword-like pattern: a long blade, a cross-guard, and a hilt.
Xerneas has the power to share eternal life; this occurs as the horns on its head shine in seven colors. When Xerneas is at the end of its life, it releases all of its life energy into all the living things in around it and transforms into a tree to sleep for 1,000 years. Xerneas also has the power to create forests, making Xerneas a benevolent force in nature. Xerneas is the only known Pokémon capable of learning the move Geomancy.
A Xerneas appeared in The Legend of X, Y, and Z! during a flashback, where Jan asked for its help after Yveltal caused trouble upon his kingdom, including turning Aila to stone. Though it restored much of the damage Yveltal inflicted, it could not revive Aila, and thus left.
Xerneas debuted in An X-cuse to Come Out and Play, alongside Yveltal. The two faced each other in a battle, which caused chaos and destruction in Vaniville Town. The two eventually stopped fighting and returned to their dormant tree and cocoon states, which were sought after by Team Flare to power the ultimate weapon. Later Xerneas awakened and took back most of the energy that had been absorbed from it. Afterwards, it was befriended and caught by Y, who nicknamed it Xerxer. When Malva arrived and revealed that she had captured Yveltal, the two nearly clashed again before Xerosic stopped them, stating that Lysandre ordered a ceasefire, as the powers of the two would cause an endless war. Xerxer agreed with this and escaped carrying Y and her friends. During the final battle against Team Flare, Xerxer and Yveltal clashed again, and eventually chose to return to their dormant states in order to prevent Lysandre from using Zygarde to his own deeds.
Xerneas appears as a Poké Ball Pokémon. In the 3DS version, it is unlocked after playing Target Blast at least once, while in the Wii U version, it is unlocked by breaking at least 200 blocks in a solo game of Trophy Rush as Pikachu. When released, it uses Geomancy to energize nearby fighters, giving their attacks stronger knockback for a time or giving them stronger defenses.
NA:The horns on Xerneas's head give this Fairy-type Pokémon the power to bestow eternal life. It emanates energy with a move called Geomancy that causes all the fighters to glow. As pretty as it is, this makes everyone more easily launchable. Know when to take advantage and when to lay low.
PAL: The horns on Xerneas's head give this Pokémon the power to bestow eternal life. When summoned in a match, energy will emanate from it and make all fighters glow. Pretty it may be, but this Geomancy move makes everyone much easier to launch. Knowing that, will you take advantage of the situation, or try to stay out of the way?
Xerneas has the same stat distribution as Yveltal.
Xerneas, along with Yveltal, was designed by Ken Sugimori and Yusuke Ohmura. Initial design for the two Legendary Pokémon was started by Sugimori, but he encountered "artist's block". For the first time, he handed the work over to another designer, Ohmura, who furthered the designs, after which Sugimori returned to finish them.
Xerneas's Neutral Mode has no special animations that play in the Hoenn Pokédex, but it plays Active Mode's attack animations if tapped on in the Pokémon summary screen.
When a Shiny Xerneas is obtained by event, its model on the Mystery Gift screen is incomplete. It appears in its Active Mode, except the multicolored additions to its horns are either missing or the same blue color as on its neck.
Xerneas's appearance is based on a cervid (more specifically the extinct species Eucladoceros) in the shape of the letter X (known as a saltire in heraldry) or an X mark.