Saiken

The is a tailed beast sealed within Utakata of Kirigakure. It has been captured by Nagato in the anime, then extracted and sealed by Akatsuki. It was later resealed into a resurrected Utakata during the Fourth Shinobi World War.

Background
According to Tobi, the Six-Tails first 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 Six-Tails eventually fell into the possession of Kirigakure and was sealed into Utakata.

According to the anime, at some point before his appearance, Utakata's master, Harusame, tried and failed to remove the Six-Tails from within Utakata to save his student from being a living weapon for his 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 the Six-Tails and killing Harusame.

Appearance
The Six-Tails resembles a white, chubby, bipedal slug with small arms and feet. It has two prominent optical tentacles (eyes) and hole-like openings as a mouth. Its entire body is covered in a slimy substance.

Abilities
Not much is known about the Six-Tails but since it is a tailed beast, it has a great amount of chakra and it is able to use it to create the Tailed Beast Ball. It is also able to emit corrosive, alkali substances that are extremely dangerous on contact in the form of a liquid or a gas, which was able to burn through the skin of the Eight-Tails' hand. The gaseous form was able to instantly disintegrate a tree that came into contact with it.

Tsuchigumo Kinjutsu Arc
In the manga, the Six-Tails' capture is not seen. In the anime, before Taka began working with Akatsuki, Akatsuki was said to have seven tailed beasts. When Taka is sent to capture the Eight-Tails, the group has only six, the seventh missing beast being the Six-Tails. Utakata's capture is the subject of its own arc, though little is actually seen of the Six-Tails outside of Utakata's transformations. Utakata is captured by the Six Paths of Pain at the end of the arc and the Six-Tails is extracted from his body, resulting in his death.

Shinobi World War Arc
After Utakata's full transformation into the Six-Tails, he attacks Kakashi and Guy, emitting a corrosive alkali from its mouth, which Kakashi warns Guy not to touch. After Guy dissipates the gas with his Morning Peacock technique, the Six-Tails prepares to attack him again. After Son Gokū is resealed into the Demonic Statue of the Outer Path, Tobi, who is intent on going all-out from this point on, forces Utakata to fully transform into the Six-Tails.