Saiken

The Six-Tailed Giant Slug (六尾の蛞蝓, Rokubi no Namekuji) is a tailed beast sealed within Rokubi Okibiuka, but was formerly sealed within Ukataka of Kirigakure. It had been captured, extracted, and sealed by Akatsuki, but was sealed within Rokubi right after Akatsuki's fall.

Background
According to Madara Uchiha, the Six-Tails came into being in the waning days of the Sage of the Six Paths' life; to prevent its resurgence, the Sage split up the chakra of the Ten-Tailed Beast into nine separate, monstrous constructs of living chakra, which would become known as the tailed beasts. The Six-Tails eventually fell into the possession of two unknown teenagers, who had discovered the Gedō Mazo, and was sealed into Rokubi.

Appearance
The Six-Tails resembles a white, bipedal slug with small arms. Its entire body is covered in a slimey substance.

Activity Inside Rokubi
Only appearing very few times in the story, The Six-Tails, has not, as of yet, become a particularly interesting character. Rokubi has used it once or twice to strike those who cause him much pain, but, other than that, we have not seen very much when it comes to the Slug. Like Naruto, though, he seems to be very ferocious once unleashing a bit of its chakra. It is unknown, however, if he can control this beast.