Gyūki

The is an ushi-oni demon sealed within Kirābī of Kumogakure.

Background
The Eight-Tails is currently one of the two remaining Tailed Beasts that were not extracted by Akatsuki, the other one being the Nine-Tailed Demon Fox.

When Team Hawk arrived to Kumogakure in 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 Team Hawk. 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 activated the Amaterasu on the Eight-Tails and it fell into pain by its flames. Kirābī then was captured by Team Hawk and was given to Madara Uchiha. His brother, the Raikage, swore that the Akatsuki would pay for what they had done.

Activity inside Kirābī
Little is known about the Eight-Tails' strength so far. As the Eight-Tails' host, Kirābī can transform himself into the Eight-Tails and can access to its high-level chakra (possibly almost in the same league as the demon fox's level) without losing control. It has been shown to fire an attack similar to the Four-Tailed Strong Ball, but with much greater force.

Kirābī is able to use the Eight-Tail as a partner to block his chakra to break illusions, even ones casted from a Mangekyo Sharingan.

Influence
An, is a creature which appears in the folklore of Japan. An ushi-oni is a massive, brutal sea-monster with a head and body of a bull with eight tentacles of an octopus, which lives off the coast of Western 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 are 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
As most of the other Tailed Beasts seen so far are based upon mythological creatures, it was speculated that this Tailed Beast was based upon the eight-headed and eight-tailed dragon, Yamata no Orochi. Some fans believed that Orochimaru was the Eight-Tails due to his snake-like abilities, however, this was proven false in chapter 408. In chapter 413, it has been confirmed that that the Eight-Tails is a giant bull with eight tentacles for it's tails, which is called an Ushi-Oni.

The Eight-Tails has only one complete horn, the other is halfway missing. Some fans believe this may be a battle scar left by Kirābī's brother, who was mentioned by Kirābī when Sasuke dodged an attack while Kirābī was under the Eight-Tails' chakra cloak.