Ah-Un
Ah-Un (阿吽 Aun) is a two-headed dragon yōkai in the manga and anime series InuYasha.
Apparently, even though he has been traveling with Sesshomaru for centuries and has been a loyal/domesticated yōkai, he didn't have a name until Rin gave him one. He is one of the only two yōkai in the entire series who is revealed to be a herbivore, the other being Totosai's three-eyed cow demon.
He has brown scales all over his body and is the closest thing in Sesshomaru's retinue to a working animal; his right head is Ah and his left head is Un. On their journeys, Rin and Jaken often rest on his back when they are flying over great distances. When Sesshomaru leaves Rin in a safe place, either with Jaken or by herself, Ah-Un is always with her. Rin has beseeched Sesshomaru to 'come back and get us' when he has taken Jaken with him on a potentially dangerous journey; a sign that Rin considers Ah-Un part of her strange new family on the same level with herself and Jaken.
Ah-Un can fire yōkai energy from both mouths. From the right mouth he fires blue beams of lightning. He's never used his left mouth for offensive purposes; but it does shoot green lightning which can control clouds and likely allows him to control the weather. With this ability, Ah-Un saved Rin from lesser demons when she was collecting an herb which can save Jaken from the poisons of Naraku's Saimyōshō; he also defeated many of Setsuna no Takemaru's zombie warriors in Inuyasha the Movie: Swords of an Honorable Ruler.
There are some questions as to the actual spelling of the creature's name and it has appeared various places as Aa-Un, Ah-Un, and Aun all of which work with Rin's pronunciation of the name. Most likely, Ah-Un takes its name from the Nio (Benevolent Kings), a pair of protectors who stand guard outside the temple gate at most Japanese Buddhist temples, one on either side of the entrance. Their fierce and threatening appearance wards off evil spirits and keeps the temple ground free of demons and thieves. Each is named after a particular cosmic sound. The open-mouthed figure is called "Agyo," who is uttering the sound "ah," meaning birth. His closed-mouth partner is called "Ungyo," who sounds "un" or "om," meaning death. Other explanations for the open/closed mouth include: (1) mouth open to scare off demons, closed to shelter/keep in the good spirits; (2) "Ah" is the first letter in the Sanskrit alphabet and "Un" is the last (same in Japanese syllabary too), so the combination symbolically represents all possible outcomes (from alpha to omega) in the cosmic dance of existence. The name could also mean loosely "yes-yes" in English.