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.

As it's Jinchūriki, Kirābī, battled Team Hawk to the point of annoyance. With his great control over the beast, Kirābī transformed himself to the Eight-Tails, and proceeded to terrorize Team Hawk. Sasuke Defeated Kirabi in chapter 415 with Amaterasu.

Abilities
Little is known about the Eight-Tails' strength so far, and his chakra levels are noted to be insanely high, possibly even in the same league as the Nine-Tails' level. It has been shown to fire an attack similar to the Four-Tailed Strong Ball, but with much greater force since he is in his Eight-Tailed state.

Kirābī is able to use the Eight-Tails' chakra to break illusions, even ones casted from a Mangekyo Sharingan. He also has control over his inner demon.

Influence
The, is a creature which appears in the folklore of Japan. The 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.