Saiken

Saiken is an enormous white — with a light blueish tint — bipedal slug with stubby arms, feet and six long tails. It has two prominent optical tentacles (eyes) and hole-like openings as a mouth. Its entire body is covered in a thick, slimy substance. When seen during the last days of the Sage of Six Paths' life, Saiken was much smaller than its present-day self, but was still much larger than the Sage.

Saiken 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 Saiken to live in a temple built for the purpose of protecting it in a humid cavernous region. During the First Shinobi World War, Hashirama Senju captured eight of the tailed beasts and sold Saiken and Isobu to Kirigakure at the Five Kage Summit, as a sign of friendship and to ensure a balance of power amongst their new villages. The tailed beast would end up being sealed within Utakata decades later. In the anime, Harusame tried and failed to remove Saiken from within his student, intending to save his Utakata from being a living weapon for their village. Utakata, however, having not heard what his master had said to him, later came under the assumption after the incident that his master had tried to kill him. This failed removal resulted in Utakata transforming into Saiken and killing Harusame.