Gyūki

The is a demon ushi-oni sealed within Kirābī of Kumogakure. It is also the only Tailed Beast not captured by Akatsuki,alongside the Nine-Tailed Demon Fox.

Personality
Kirābī and the Eight-Tails appear to be able to speak to each other in a much more relaxed state than Naruto and the Nine-Tailed Demon Fox. For example, when Naruto has a conversation with the demon fox, the latter is in a cage with a special seal keeping it from taking over Naruto. Kirābī and the Eight-Tails, however, talk without such restraints, with Kirābī even sitting on top of its muzzle. The Eight-Tails is shown to get easily annoyed with Kirābī's crazy antics, however, that is somehow shown to be comical.

Hunt for the Eight-Tails
When Taka arrived at Kumogakure in an attempt to capture Kirābī, he battled them to the point of annoyance. With his great control over his demon, Kirābī transformed himself into the Eight-Tails, and proceeded to terrorize Taka. As the Eight-Tails attacked, Suigetsu commanded a wave of water, attempting to defeat it. The Eight-Tails understood that Suigetsu had the advantage given that they were surrounded by water, so it then charged a huge ball of chakra and fired it at Suigetsu, defeating him.

Later, Sasuke Uchiha activated his Mangekyo Sharingan and released Amaterasu on the Eight-Tails and it fell into pain by its flames. Kirābī was then seemingly captured by Taka and was given to Madara Uchiha. Kirābī's brother, the Raikage, swore that Akatsuki would pay for what they had done.

It was later revealed that Kirābī faked his capture. After Taka left, he and the Eight-Tails left the lake and Kirābī took this opportunity to take a "vacation" from being a guardian. The Eight-Tails commented that a new era is coming, and Kirābī thinks its going to be his singing career.

Activity inside Kirābī
Little is known about the Eight-Tails' strength so far. As the Eight-Tails' Jinchūriki, Kirābī can transform himself into the Eight-Tails and can access its high-level chakra without losing control. It has been shown to fire an attack similar to the Four-Tailed Fox Menacing Ball, but with much greater force. Kirābī is also able to use the Eight-Tails to block his chakra to break illusions, even ones cast from a Mangekyo Sharingan.

Influence
An is a creature which appears in Japanese folklore. An ushi-oni is a massive, brutal sea-monster with the horned head of a bull and the body of a sea animal, like an octopus or a crab. It lives off the coasts of Japan and attacks fishermen.

An ushi-oni appears as protective symbol in the Ushi-oni-matsuri, which is held in late July in Uwajima of Ehime Prefecture. Like the dragon dancers at a Chinese New Year celebration, an ushi-oni is represented with a huge, multiple-person costume with a cloth body and a carved, painted head held upon a pole. It has a sword for a tail, and is thought to drive away evil spirits.

Speculation
Before its debut, some fans believed that the Eight-Tails would be based on the Yamata no Orochi, taking the appearance of an eight-headed serpent or dragon, which also led fans to believe it would somehow be connected to Orochimaru.

Trivia
The Eight-Tails' "tails" are really the eight arms of an octopus.