A-Un

is Sesshōmaru's beast of burden. Despite traveling with Sesshōmaru for centuries, he has no name until Rin gives him one, calling the right head "A" and the left head "Un".

On their journeys, Rin and Jaken often rest on his back to fly over great distances. When Sesshōmaru leaves Rin in a safe place, A-Un stays with her as her protector. He saves Rin from a group of lesser demons while she is collecting herbs that will save Jaken from Saimyōshō venom. A-Un also defeats many of Takemaru of Setsuna's zombie warriors in InuYasha the Movie: Swords of an Honorable Ruler.

Personality
A-Un is quite a benevolent and laid-back creature, subservient to Sesshōmaru and even to Kohaku, Rin (although this could be due to the kindness Rin has shown the creature) while only occasionally obeying Jaken, who sometimes has trouble motivating the demon to move; A-Un was implied to actually have little to no respect for Jaken in the 3rd movie when Jaken encouraged Sesshōmaru not to team up with InuYasha, prompting everyone (A-Un apparently included) to rearrange his face to change his mind, showing A-Un is much more intelligent than he looks and that he could understand the situation at hand. This also showed when he merited the two dog brothers' teamwork. A-Un's benevolent nature may be due to the fact that he is one of the only two yōkai in the series who is explicitly stated to be an herbivore. A-Un is intelligent and is a sentient being; he also seems to have a sense of humor. The closest counterpart to A-Un in the series in terms of personality would be Kirara, a demon who also cannot speak but does possess a great deal of intelligence and loyalty.

Powers & Abilities

 * Flight: A-Un possesses the ability of flight in a manner similar to Tōtōsai's three-eyed cow yōkai Mō-Mō - an ability which is used to the advantage of Sesshōmaru when it comes to traversing great distances. A-Un typically carries Rin around on its back as she is not fast enough and does not possess the stamina to keep up with Sesshōmaru when they travel.
 * Lighting Blasts: A-Un can shoot bursts of energy similar to lightning bolts out of its mouths. From the right head, "A", it fires beams of blue lightning to attack its enemies when it is roused into battle. The left head, "Un", fires green lightning which allows him to control clouds and possibly the weather. However, this ability has never been used to directly attack anything and it’s possible that it isn’t harmful.

Relationships
The Dog Demon Lord in control of A-Un. A-Un has been traveling with Sesshōmaru as a servant for a long time, longer than his vassal Jaken and especially longer than his most recent traveling companion, Rin.
 * Sesshōmaru

The young human girl traveling with Sesshōmaru has a special relationship with A-Un. Rin named the demon and cares for him, as such A-Un looks after Rin in turn, protecting her and allowing her to ride on his back. A-Un has saved Rin on multiple occasions. The two most memorable examples are when he saved her from some lesser yōkai while she is collecting herbs to save Jaken when he is poisoned by Naraku’s miasma, and when he protected her from the zombie warriors in the third InuYasha movie, Swords of an Honorable Ruler. If Rin is in danger in his presence, he will do everything he can possibly do in order to protect her.
 * Rin

The demon vassal of Sesshōmaru had a somewhat strained relationship with A-Un, apparently struggling to control the demon, unlike Sesshōmaru, Rin and Kohaku who A-Un willingly obeyed without question.
 * Jaken

It seemed that A-Un had no trouble being led by Kohaku when he joined the group and let him ride on his back all the time.
 * Kohaku

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 * Setsuna

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 * Towa Higurashi

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 * Moroha

Trivia

 * It is undecided whether A-Un's name can be translated into anything applicable to their character, but the pronunciation a-un is used to describe a sound influential in many Indian religions "indicating an inherently harmonious relationship or non-verbal communication" . Also, a-un is a term typically used to identify the 'one open mouth and one closed mouth' komainu statue sets normally seen near or in shrines.
 * A-Un is one of the only two yōkai in the series who is explicitly stated to be an herbivore (the other being Mō-Mō).
 * "A-Un" may be in reference to a pair of stone gargoyles (or Koma-inu, Nio Protectors of Japan): Agyo and Ungyo. They're usually represented with one having the mouth open and the other having its mouth closed. They're protectors against holy spirits, which could explain A-Un's benevolant nature. Koma-inu craftsman in Japan refer to Agyo and Ungyo as "A-Un" for short.

Hanyō no Yashahime

 * Episode 20 (Hanyō no Yashahime)