Isobu

The is a gigantic, turtle-like demon that Akatsuki assigns Tobi (with the help of Deidara) to capture.

Background
The demon turle lived in an ocean area and has remained uncaptured by any shinobi until Deidara and Tobi arrived. Right after the demon turtle emerged from the water, it quickly swam towards Tobi, chasing him around area. With the demon turtle distracted, Deidara molded an explosive clay fish in an attempt to capture it. It's unclear if this marked the end of the battle, as the remainder of the battle goes unseen. As the two towed the Tailed Beast to the Akatsuki's lair after the battle, Tobi gloated of bringing the demon turtle down with his special jutsu, but Deidara thought otherwise. It is later sealed in the sealing statue.

Unlike the other Tailed Beasts seen this far, the demon turtle lacked a host. According to Deidara, lacking a host made the demon turtle weaker because it wasn't intelligent enough to control its own strength.

Abilities
What abilities this creature possesses remains unclear, as it did little to actually attack the two in its short appearance. It is clearly quite fast despite its size, as evidenced by its ability to keep up with Tobi, who was running on the water at the time.