Gyūki

The is a tailed beast sealed within Killer B 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 his ability 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 that 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 the Kohaku no Jōhei. 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 B, who was five at the time, through the Iron Armour Seal. B would go on to tame and control the Eight-Tails through rigorous training within the Falls of Truth.

At some point in the past, one Kumo-nin attempted to mimic the Gold and Silver Brothers by eating one of the Eight-Tails' chakra meat tentacles in the hopes of gaining the power like a jinchūriki, but died in process.

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 lower 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, instead its lower half is made up of its tails which consist of eight tentacles that resemble the cephalopod arms of an octopus, and according to Killer B, its tails will grow back in the event that they are sliced off. It is also extremely massive, matching the Giant Squid in size.

Personality
Since Killer B is one of the few jinchūriki to gain control over his tailed beast, his relationship with the Eight-Tails is very relaxed. Where the Nine-Tailed Demon Fox constantly threatens its jinchūriki and must be either restrained or confined to be spoken to, B and the Eight-Tails can talk leisurely, with Killer B even sitting on top of its muzzle. However, B tends to annoy the Eight-Tails to comical effect; his serious and no-nonsense demeanour often contrasts with B's tendency to goof off whenever he likes, adding fuel to the Tailed Beast's irritation to his vessel. The Eight-Tails is not above bad-mouthing B as a result. The Eight-Tails appears to be much more worthy than B as far as responsibility is concerned, having urged his jinchūriki to follow Naruto after the boy rose up to supposedly "take a leak". Before B learned to control it, the Eight-Tails was infamous to be wild, violent, and went on many rampages when its former jinchūriki would lose control over it.

Despite the Eight-Tails' friendly relationship with Killer B, 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 B 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 B changed it. It also said that it doesn't like the Nine-Tails but insisted that it was no reason for B to refuse helping Naruto take control over it, hinting a benevolent side.

Abilities
The Eight-Tails was said by Kisame to be the second strongest of the tailed beasts. With its great supply of chakra, it can fire massive chakra blasts. 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 Tailed Beast Ball from the Nine-Tails, but was knocked out of the battle afterwards because of it. It is able to endure being burned by Amaterasu for a considerable amount of time. Its tentacles offer it several additional appendages with which to interact with its surroundings and strike targets, but they are also easily severed. Even with its immense size, it has shown a high level of speed when it attacks, and even fast enough to block Amaterasu with its fist. A single Tailed Beast Ball from it could penetrate nine of the thirty-six barriers without even exploding. Going with its octopus theme, it can also produce ink to blind opponents.

The Eight-Tails has been shown to be very intelligent, explaining the mechanics of the Tailed Beast Ball to Naruto and the weaknesses of his Nine-Tails Chakra Mode. The Eight-Tails even coached B in tactics on how to counter Kisame's water-based techniques. He later worked out that the Gold and Silver Brothers were revived, after witnessing Naruto sense the Nine-Tails' chakra coming from outside of the training place that he and B were confined, correctly deducing that Orochimaru's Impure World Resurrection was being used by someone participating the war.

Activity inside Killer B
As the Eight-Tails' jinchūriki, Killer B can transform himself into the Eight-Tails and can access its high-level chakra without losing control, as Killer B is one of few jinchūriki who is known to be able to fully control his tailed beast's powers. Killer B 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 B, he battled them to the point of annoyance. With his great control over his demon, Killer B 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 B disguised himself as the cut part of the tail and escaped. Killer B was then seemingly captured by Taka and was given to Madara Uchiha. Killer B's brother, the Fourth Raikage, swore that Akatsuki would pay for what they had done.

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

Five Kage Summit Arc
As Kisame attacked Killer B, the Eight-Tails demanded B to let him out. However, B reminded him that he would just kill Sabu and Ponta in the process of protecting itself. With this, Eight-Tails begrudgingly let B use the version 2 form, as well as giving B advice during the battle. However, as Kisame knocked B out, the Eight-Tails yelled at him to wake up, worried as both B and it were nearly out of chakra. Kisame then prepared to finish the battle as the Raikage arrived and helped B 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 B, in his full Eight-Tails form, forced the squid back and saved the ship. Upon seeing it, Naruto realized that the Eight-Tails and B were the "octopus" he's been looking for to guide him to control the Nine-Tailed Demon Fox. After B was done training with some bears in the forest, the Eight-Tails tried to convince B to train Naruto, stating that even though it wasn't fond of the Nine-Tails, it still thought B should train him since they are both jinchūriki.

Shinobi World War Arc
When Naruto failed to fully transform into the Nine-Tails, the Eight-Tails requested that B let him speak with Naruto. He explained the risks of using a tailed beast's chakra, and how using it meant that the Nine-Tails could deplete Naruto's chakra, because when Naruto uses the Nine-Tails's chakra, it can take his. It explains that Naruto won't be able to transform fully until he can reach an agreement with the Nine-Tails, which it recognises it's unlikely, noting that it and B used to be like that. It also says that because of that, he won't be able to learn the Tailed Beast Ball. When it is done, B changes back to his normal self.

He is later seen surprised at how Naruto was able to discern the chakra coming from Kinkaku and Ginkaku despite being secured in the training zone.

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.

Trivia

 * All known jinchūriki of the Eight-Tails were from the same family.