Gyūki (Zf6hellion)

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-Tails' 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 another selected host. Many villagers lost their lives during these episodes and the Eight-Tails lost one of its four horns to the Raikage. It was ultimately sealed within Tenka Nihisaki, who was six at the time, and personally requested that she become the host - although she didn't truly understand the situation at the time or her role in life after it, through the Iron Armour Seal. Tenka would go on to found a partnership with the Eight-Tails after she betrayed Kumogakure and fled the Land of Lightning.

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 the Raikage. 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 former hosts, its tails will grow back in the event that they are sliced off. It is also extremely massive, matching the Giant Squid in size.