Gyūki

The is a demon ushi-oni sealed within Killer Bee of Kumogakure.

Background
According to Madara Uchiha, the Eight-Tails came into being near the end of the waning days of the Sage of the Six Paths' life; to prevent its resurgence, the Sage split up the demon's chakra of the Ten-Tailed Beast into nine separate, monstrous constructs of living chakra, which would become known as the tailed beasts. The Eight-Tails eventually fell into the possession of Kumogakure and was sealed into Killer Bee.

Personality
Killer Bee and the Eight-Tails appear to be able to speak to each other in a much more relaxed state than Naruto Uzumaki and the Nine-Tailed Demon Fox. For example, when Naruto has a conversation with the Nine-Tails, the latter is in a cage with a special seal keeping it from taking over Naruto. As Killer Bee is one of the few people who has total control over his own tailed beast, he and the Eight-Tails can talk without such restraints, with Killer Bee even sitting on top of its muzzle. The Eight-Tails is shown to get easily annoyed with Killer Bee's crazy antics; however, that is somehow shown to be comical.

However, despite Eight-Tails' friendly relationship with Killer Bee, the Eight-Tails, like the other beasts, main goal is self preservation. For example, it was more than willing to be released and destroy Kisame Hoshigaki despite Sabu and Ponta being close by. However, behind the brutal appearance the Eight-Tails is shown to have a good analytical ability, as he told Killer Bee that if he left Ponta and Sabu and run in the opposite direction Kisame and the water will follow him and they (Sabu and Ponta) will be free.

Activity inside Killer Bee
Little is known about the Eight-Tails' strength so far. As the Eight-Tails' jinchūriki, Killer Bee can transform himself into the Eight-Tails and can access its high-level chakra without losing control. Killer Bee is one of the two jinchūriki who has been known to be able to fully control his tailed beast's powers, the other being Yagura. It has been shown to fire an attack similar to the Four-Tailed Fox Menacing Ball, but with much greater force. Killer Bee is also able to use the Eight-Tails to block his chakra to break illusions, even ones cast from a Mangekyō Sharingan. Killer Bee can also create ink because of the octopus nature that his tailed beast resembles.

Hunt for the Eight-Tails
When Taka arrived at Kumogakure in an attempt to capture Killer Bee, he battled them to the point of annoyance. With his great control over his demon, Killer Bee 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 Mangekyō Sharingan and released Amaterasu on the Eight-Tails and it fell into pain by its flames. As one of its tails was about to fall on Karin, Sasuke cut it with Chidori Sharp Spear. Killer Bee disguised himself as the cut part of the tail and escaped. Killer Bee was then seemingly captured by Taka and was given to Madara Uchiha. Killer Bee's brother, the Fourth Raikage, swore that Akatsuki would pay for what they had done.

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

Kage Summit arc
As Kisame attacks Killer Bee, Eight-Tails demands Bee to let him out. However, Bee reminds him that he would just kill Sabu and Ponta in the process of protecting itself. With this, Eight-Tails begrudingly let Bee use the Seven-Tailed version 2 mode, as well as giving Bee advice during the battle. However, as Kisame knocks Killer Bee out, Eight-Tails yells at him to wake up, worried as both Bee and it were nearly out of chakra. Kisame then prepared to finish the battle as the Raikage arrived and helped Bee kill Kisame.

Influence
In Japanese folklore, an 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.

Trivia

 * 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.
 * The Eight-Tails' "tails" are really eight cephalopod arms.
 * According to Killer Bee, Eight-Tails' missing tails will grow back eventually.