Gyūki

The is a tailed beast sealed within Killer Bee of Kumogakure.

Background
According to Madara Uchiha, the Eight-Tails came into being in the waning days of the Sage of the Six Paths' life; to prevent the Ten-Tailed Beast's resurgence, the Sage split up its chakra and used Izanagi to create 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.

However, its ferocity was so great, none of its former jinchūriki could control it. They would lose control and the Eight-Tails would go on a rampage and destroy the village, requiring the Third Raikage to form a special coalition to subdue it, extract it from its previous jinchūriki, and sealed it within a special pot. Many villagers lost their lives during these episodes and the Eight-Tails lost one of its four horns to A after the said horn pierced Motoi's father. It was ultimately sealed within Killer Bee, who was five at the time, through the Iron Armor Seal. Bee would go on to tame and control the Eight-Tails through rigorous training within the Falls of Truth.

Appearance
The Eight-Tails resembles an ushi-oni with four long horns on its head, similar to that of a Jacob sheep. It also has a fifth horn like protrusion on its chin, which appears to have been damaged, much like its upper left horn, part of which was sliced off in battle by A. It has a muscular upper-body structure, with a hunched back similar to an American bison, arms with spiked protrusions on the elbows, and hands with opposable thumbs like that of a human. It has no hind-legs. Its tails consist of eight tentacles that resemble the cephalopod arms of an octopus, and according to Killer Bee, its tails will grow back in the event that they are sliced off.

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 only shown to be comical. However, before Bee learned to control it, the Eight-Tails was wild, violent, and went on many rampages when its former jinchūriki would lose control over it.

However, despite Eight-Tails' friendly relationship with Killer Bee, the Eight-Tails' main goal, like the other beasts, 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 it 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. The Eight-Tails mentioned that it used to be much more aggressive and violent but that Killer Bee changed it. It also said that it doesn't like the Nine-Tails but insisted that it was no reason for Bee to refuse helping Naruto take control over it, hinting a benevolent side.

Abilities
With its great supply of chakra, the Eight-Tails can fire massive chakra blasts, which seem to be made of black lightning. During Naruto's battle with the Nine-Tails within his subconscious, it was shown that the Eight-Tails had enough strength to temporarily restrain and contain a direct blast from the Nine-Tails. However, despite its immense size, it does not withstand the secondary effects of powerful attacks just as much as the Nine-Tails can, as Sasuke Uchiha's Chidori Sharp Spear and Amaterasu techniques proved effective against it, however, due to Killer Bee throwing the fight, we don't know if he weakened the Eight-Tail's body to get away. Due to its resemblance of an octopus, it can also use ink.

Activity inside Killer Bee
As the Eight-Tails' jinchūriki, Killer Bee can transform himself into the Eight-Tails and can access its high-level chakra without losing control, as 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. Killer Bee is also able to use the Eight-Tails to block his chakra to break illusions, even ones cast from a Mangekyō Sharingan.

Invasion of Pain Arc
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 blast of chakra and fired it at Suigetsu, defeating him.

Later, Sasuke 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. As the Eight-Tails commented that a new era is coming, and Killer Bee thought it was going to be his singing career.

Five Kage Summit Arc
As Kisame attacked Killer Bee, the Eight-Tails demanded Bee to let him out. However, Bee reminded him that he would just kill Sabu and Ponta in the process of protecting itself. With this, Eight-Tails begrudgingly let Bee use the version 2 form, as well as giving Bee advice during the battle. However, as Kisame knocked Bee out, the Eight-Tails yelled 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".

Confining the Jinchūriki Arc
As Naruto Uzumaki and Yamato entered the Land of Lightning, in the Allied Shinobi Forces's efforts to keep the jinchūriki away from Akatsuki, they were attacked by a giant squid. Killer Bee, in his full Eight-Tails form, forced the squid back and saved the ship. Upon seeing it, Naruto realized that the Eight-Tails and Bee were the "octopus" he's been looking for to guide him to control the Nine-Tailed Demon Fox. After Bee was done training with some bears in the forest, the Eight-Tails tried to convince Bee to train Naruto, stating that even though it wasn't fond of the Nine-Tails, it still thought Bee should train him since they are both jinchūriki.

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.