Saiken (犀犬, Saiken), more commonly known as the Six-Tails (六尾, Rokubi), is one of the nine tailed beasts. It was last sealed within Utakata of Kirigakure.
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 uses "ore" (オレ) when referring to itself, "ore" being a boastful way of saying "I". It also speaks with a dialect which may possibly be Ōsakaben in origin. In the anime, Saiken also seemed to be kinder and not as self-serving and malicious as most other tailed beasts, as when Utakata asked for its help to save Hotaru it seemed to willingly comply; and later it simply receded back into Utakata without trying to take him over. However it was not above killing for self preservation, as seen when it killed Harusame when he attempted to extract it from its jinchūriki. In the anime, it speaks with a high pitched voice and appears to be very energetic.