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.

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.