Isobu

The is a demon turtle that the Akatsuki assigned Tobi (with the help of Deidara) to capture.

Background
In the past, the Three-Tailed Beast was imbued with an unnamed shinobi from Kirigakure, later it seemed to have been extracted from its host for reasons unclear, and was set free in the wild. After which, the Turtle lived in a lake and remained uncaptured by any shinobi.

Appearence in Manga
When Deidara and Tobi arrived at the lake, the turtle emerged from the water and it quickly swam towards Tobi, chasing him around the area. With the 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 turtle to the Akatsuki's lair after the battle, Tobi gloated of bringing the Three-Tails down with his special jutsu, but Deidara thought otherwise. It is later sealed in the sealing statue.

Since it was removed from its host, the Three-Tails lost its intelligence after it had been extracted and wasn't smart enough to control it's own strength, which made it weaker and then was easily captured and sealed.

Appearence in Anime
During the second Shippuden filler arc, a boy named Yukimaru, who, based on his appearence, is believed to be a relative of the Three-Tails' host, has the unique ability to control the massive turtle (or at the very least summon it). While on a mission for Orochimaru, Yukimaru and Kabuto Yakushi traveled to a distant swamp. When Kabuto forced Yukimaru to release all his chakra, a large silluette greatly resembling the Three-tails emerged from the swamp partially before sinking back down. Needless to say, Kabuto was very pleased with the results.

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.

Influence
Turtles are often used as the base design for sea monsters in generic Japanese folklore. The Three-Tails' design in particular was conceived as a result of series creator Masashi Kishimoto's interest in monsters, yōkai, and kaijū, mixing together various elements of those types of creatures to create a monster with a turtle's body and a shrimp's tails.