Isobu

Isobu primarily resembles a large turtle, but with a crab-like shell, spikes all over its body, and three shrimp-like tails. Under its shell, it has red, muscle-like tissue. It has a pair of human-like arms and hands, but no hind-legs. His face is concealed between a large forehead and lower jaw — both of which have spikes. Its eyes are red and have crimson pupils. In the anime, the eyes are red with yellow pupils. Its right eye is constantly closed. In the anime, Guren speculated this was because its eyes were the only soft parts of its body, being heavily-armoured, and thus kept one eye closed to cover more vulnerable spots. During the last remaining days of the Sage of Six Paths, Isobu was essentially much the same in appearance, albeit smaller in size with shorter spikes around its body, but was still many times larger than the Sage.

Isobu 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 Isobu to live in a temple built for the purpose of protecting it in a region surrounding a large misty lake that would later become part of the Land of Water. During the First Shinobi World War, Hashirama Senju captured eight of the tailed beasts and sold Isobu and Saiken to Kirigakure at the Five Kage Summit, as a sign of friendship and to ensure a balance of power amongst their new villages. Decades later, Kirigakure — having been manipulated by Madara Uchiha — sealed Isobu into the Konohagakure shinobi, Rin Nohara. They plotted for Rin to be taken back to her village as a Trojan horse, releasing the tailed beast to wreak havoc in Konoha. This failed, however, as Rin was fully aware of their schemes and opted to kill herself at hands of her friend and fellow teammate, Kakashi Hatake, before their objectives could come to fruition. Eventually, Isobu was revived and recaptured by Kirigakure, who sealed Isobu within Yagura, the Fourth Mizukage, who managed to gain complete control and harmony with it. However, Yagura was placed under the genjutsu of Obito Uchiha, who controlled the Mizukage, and Isobu by extension. The genjutsu was dispelled years later by Ao, which led to Yagura's death shortly after. Some time after Yagura's death, Isobu was revived once more, although it was not recaptured and therefore was free to live in the wild. In the anime, Tsunade states that Isobu disappeared, its whereabouts unknown, after the Third Shinobi World War.