Saiken

The is a demon slug sealed within Utakata of Kirigakure. It is a giant, bipedal slug with six tails. It has been captured, extracted, and sealed by Akatsuki. While the events surrounding this are currently unknown, Utakata and the Six-Tails appear in an anime filler arc.

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, but was captured and extracted by Akatsuki. 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 remain yet to be captured by Akatsuki. In order to avoid conflict with the manga, it seems inevitable that, by the end of the arc, Utakata will have been captured and the Six-Tails extracted, resulting in his previously-unseen death.
 * 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.