Gyūki

An ushi-oni with an ox's head, Gyūki carries four long horns like those of a Jacob sheep. It is colossal even by tailed-beast standards, dwarfing forests, cliffs, and towers alike. Its upper body is muscular and hunched in the manner of a bison, its elbows spiked, its hands human-like with opposable thumbs, while below the waist it has no legs, only eight octopus-like tentacle-tails that regrow when cut. Old wounds scarred it for good: A sheared away the lower left horn, while a point-blank shot from its own Tailed Beast Ball wrecked the right horns and blew out a tooth, part of which grew back after the war. During Hagoromo's last days it was smaller, with far shorter horns.

Gyūki is gruff and no-nonsense, speaking bluntly and reading a battle with a sharp analytical eye. It was once notorious throughout Kumo for rampaging whenever a host lost control, a fury rooted in its resentment at centuries of being caged, but everything changed after Killer B beat it one-on-one and the two became real friends. Their bond runs deep yet prickly, since B's goofing and crude jokes constantly try the serious beast's patience, and in truth Gyūki often acts as the more responsible of the pair. For all its dislike of Kurama's habit of ranking beasts by tail count, it still chose to aid Naruto, and even after extraction it tore off a piece of itself to keep B alive, later confessing it missed his terrible rapping.

Gyūki and its fellow beasts came to be when Hagoromo Ōtsutsuki divided the Ten-Tails into nine chakra forms, gave each a name, and foretold that one day they would be reunited and shown what true strength is. At the first Five Kage Summit, Hashirama Senju, having captured most of the beasts, presented Kumogakure with both Gyūki and Matatabi as a gesture of goodwill meant to balance power among the villages. For decades the ox cycled through Kumo's hosts, none able to tame it, and its rampages claimed many lives, until it was sealed into Killer B at the age of five, who through brutal training at the Falls of Truth beat and then befriended it.

Reckoned the second strongest of the tailed beasts, Gyūki holds a vast reserve of chakra that it shares, refills quickly, and uses to shatter powerful genjutsu. It launches the Tailed Beast Ball singly, in rapid bursts, or as a huge supercharged wave capable of leveling a valley and forcing the Ten-Tails' own attack back inside it, and it spits ink to blind or clone. Its tentacles spin into tornadoes, coil around it as a shield, and channel its chakra, even keeping B alive after extraction. Tremendously strong and surprisingly quick, it can pin Kurama, catch Amaterasu on its fist, and heal wounds within seconds, and being effectively deathless it returns in a younger body whenever slain.

The Fourth Shinobi World War saw B and Gyūki hold the front line: they drove off Taka and Kisame Hoshigaki, coached Naruto on working with a tailed beast, and battled the resurrected jinchuriki under Tobi's control. Gyūki helped fight Madara Uchiha and the revived Ten-Tails, was pulled out and sealed into the Demonic Statue but severed a tentacle so B could live, and later fed its chakra to Naruto by way of Obito. Once Kaguya Ōtsutsuki fell and the beasts were freed, Gyūki startled Kurama by choosing to go back to B.

Once the war ended, Gyūki was sealed back into B, only to be seized later by Momoshiki and Kinshiki Ōtsutsuki, who drained its chakra and left an unconscious B to drown, though both pulled through. In the anime it took part in the beasts' telepathic meetings once Urashiki Ōtsutsuki began hunting their chakra, and it felt Kurama perish when the fox gave its life against Isshiki Ōtsutsuki.