Saiken

The is a tailed beast that was sealed within Utakata of Kirigakure. It was 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-Tailed Slug 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-Tailed Slug and killing Harusame.

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
The anime depicts these events in a filler arc, which also shows that, sometime prior to the story, Utakata's former master, Harusame, attempted and failed to remove the Six-Tails from within Utakata. Because Utakata and Kirigakure ANBU captain Tsurugi have differing opinions on what happened, it is unknown whether Harusame was trying to kill Utakata or save him. Whatever the case, Harusame was killed when his attempt to remove the Six-Tails resulted in Utakata taking on its form. The death of Utakata's master led to his flight from Kirigakure.

Trivia

 * In the anime, Utakata and the Six-Tails were active longer before being captured by Akatsuki. In order to avoid conflict with the manga, by the end of the arc, Utakata was captured by Pain in the anime arc and the Six-Tails extracted, resulting in his previously-unseen death in the manga.
 * As noted above, by chapter 404, the manga equivalent of episode 142 of Shippuden, Akatsuki had possession of seven tailed beasts, the Six-Tails included.