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 are slated to appear in an upcoming filler arc in the anime.

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-Tailed Slug eventually fell into the possession of Kirigakure and was sealed into Utakata, but was captured and extracted by Akatsuki.

Trivia

 * In an upcoming filler arc in the anime, Utakata and the Six-Tails remain yet to be captured by Akatsuki.