Gyūki

Gyūki is an ushi-oni with four long horns on its head, similar to that of a Jacob sheep. As a tailed beast, it is extremely massive, dwarfing entire forests and towers over large rock formations and buildings and matching the Giant Squid in size. The lower left part of its horns was permanently sliced off in a battle with A, and the right horns were destroyed by its own point-blank Tailed Beast Ball when fighting the Ten-Tails. It also has straight teeth, one of which was blown off during the point-blank Tailed Beast Ball. Following the Fourth Shinobi World War, it grew another tooth, along with its horns (albeit not completely). Gyūki also 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. Instead of hind-legs, its lower half consists of its eight tails, which resemble the cephalopod arms of an octopus. These tails will grow back in the event they are sliced off. During the last remaining days of Hagoromo Ōtsutsuki, Gyūki was smaller and its horns were much shorter.

Gyūki and the tailed beasts first came into being in the waning days of Hagoromo Ōtsutsuki, who used his Creation of All Things Technique to divide the Ten-Tails' chakra into nine separate bodies and giving each a name. Some time after being created, the Sage explained to the young tailed beasts that a link existed between them despite their separation. He also foretold of a day when they would be brought together again - though not as they were - and that at that time someone would emerge to show them what true power is. In the anime, before his death, Hagoromo sent Gyūki to live in a temple built for the purpose of protecting it in a region surrounding a range of tall mountains between the clouds, that would later become part of the Land of Lightning. In an unknown time, Hashirama Senju captured eight of the tailed beasts and sold Gyūki and Matatabi to Kumogakure at the Five Kage Summit, as a sign of friendship and to ensure a balance of power amongst their new villages. In a cavern behind the Falls of Truth, where jinchūriki learn to control their tailed beasts' power, there are wall murals of Gyūki and Matatabi, which indicates how long they were associated with the country. At some point in the past, one Kumo-nin attempted to mimic the Gold and Silver Brothers by eating one of Gyūki's chakra meat tentacles in the hopes of gaining jinchūriki-like power, but died in the process. Throughout the years, none of Gyūki's former jinchūriki - such as Blue B, and his uncle and father before him - could control its power and the ox would constantly go on a rampage, destroying the village and killing many of its inhabitants. Despite these setbacks, the Third Raikage believed a jinchūriki for Gyūki was too valuable of a war deterrent and decided to keep looking for a compatible one, to which he would form a special coalition to subdue and seal it in the Kohaku no Jōhei. During one of its rampages, the Third fought Gyūki one-on-one until they both collapsed from exhaustion. During another, Gyūki had one of its four horns lost to A, the offending horn having killed Furui. In the anime, Blue B's rampage was stated to be orchestrated by Orochimaru, who gave Blue B genjutsu-inducing pills to draw out Gyūki. Due to the pills, Gyūki had no recollection of the rampage. Gyūki was ultimately sealed within Killer B, who was five at the time, through the Iron Armour Seal. Through rigorous training within the Falls of Truth, B managed to defeat and eventually befriend Gyūki.