Isobu

The is a demon turtle that the Akatsuki assigned Tobi (though Deidara did all the work) 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.

Three-Tails Arrival filler arc
During the second Shippuden filler arc, a boy named Yukimaru has the unique ability to control the massive turtle (or at the very least summon it). It has been speculated that Yukimaru may be somehow related to the Jinchuuriki pictured below, based on their common hair and eye color, as well as their common birthplace. While on a mission for Orochimaru, Yukimaru and Kabuto Yakushi traveled to the lake. When Kabuto forced Yukimaru to release all his chakra, a large silhouette greatly resembling the Three-Tails emerged from water partially before sinking back down. Needless to say, Kabuto was very pleased with the results.

Later, Kabuto returned with Yukimaru again, this time also with Guren. By having the two combined their powers, the Three-Tailed Beast finally emerged, much to Kabuto's satisfaction. Kabuto then orderes Guren to restain it. However, the turtle proves so powerful that the small-scale crystal jutsu do little more than annoy it. Ultimately, Guren is able to trap it in a large barrier of crystal, but soon escapes. The turtle's rage then finally hits its peak, where it creates a tremendous tidal wave that blasts everyone away.

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.

Abilities
What abilities this creature possesses remains unclear in the manga, as it did little to actually attack Deidara and Tobi in its short appearance in the comic. The turtle 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. While fighting the turtle, Tobi complained that Kisame Hoshigaki would've been a better choice to capture the creature beacuse of the turtle's water techniques, which suggests that it could use water techniques, despite not beeing demonstrated.

In the anime version, in addition to its incredible speed, the Turtle can create a large tidal wave capable of blasting anyone or anything in its way, proving Tobi's statement in the manga that the beast is infact a water-type.

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 tailswith a crab looking figure.

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