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.

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 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.

As a note, Utakata's statement of not wanting to use the slugs power implies, even though he has control of the six tails, he may have had times where it was able to control him. This could mean the six tails is actually violent in nature, much like the nine tailed fox, or that Utakata only has a degree of control over the slugs full form.

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

Six-Tailed Beast 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.