Saiken

The is a demon slug sealed within Utakata of Kirigakure. It is a giant, bipedal slug with six tails. It is speculative that Utakata's bubble techniques are derived from the Six-Tail's power. It has been captured, extracted, and sealed by Akatsuki.

Background
According to Madara Uchiha, the Six-Tails came into being near the end of the waning days of the Sage of the Six Paths' life; to prevent it's resurgence, the Sage split up the demon's chakra of the Ten-Tailed Beast into nine seperate, monsterous constructs of living chakra, which would become known as the 'Tailed Beasts'.