Isobu

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

Background
In the past, the Three-Tails 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 giant turtle lived in a lake and remained uncaptured by any shinobi. According to Tsunade, the Three-Tails was lost after the great war.

Three-Tails Arrival filler arc
During the second Shippūden filler arc, a boy named Yukimaru has the unique ability to control the giant turtle (or at the very least summon it). It has been speculated that Yukimaru may be somehow related to the Jinchūriki 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 giant turtle 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 giant turtle finally emerged, much to Kabuto's satisfaction. Kabuto then orders Guren to restrain 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 giant turtle's rage then finally hits its peak, where it creates a tremendous tidal wave that blasts everyone away. After somewhat settling down, the beast then creates a powerful fog that engulfs the entire area.

Capture
When Deidara and Tobi arrived at the lake, the Three-Tails emerged from the water and it quickly swam towards Tobi, chasing him around the area. With the giant 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 Three-Tails to the Akatsuki's lair after the battle, Tobi gloated of bringing the giant turtle 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 manga. The Three-Tails 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 giant turtle, Tobi complained that Kisame Hoshigaki would've been a better choice to capture the creature because of the giant turtle's affinity with the water, which suggests that it could use water techniques, despite not being demonstrated. In the anime, however, it has been shown that its skin is very hard, able to shatter Crystal Release techniques with ease, it could also unleash a powerful roar attack that can repel matter, the roar also appears to have some relation to sonics as it could shatter crystals due to the high frequency, finally it has a 360° tidal wave technique that can cause large scale damage. It has limited genjutsu capabilities that enables it to create a powerful fog that can cause animals to become confused and people to hallucinate. One other ability that the Three-Tails has been shown to use was the opening of a portal to a separate dimension within the lake that it had appeared from. Whether or not this ability is exclusive to the Three-Tailed beast or if the other Tailed creatures can use it as well is unknown. It has also not been revealed if the Tailed beasts reside within their own dimension together or separated.

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 tail with a crab looking figure.

Trivia

 * The capture of the Three-Tails in the anime was much more detailed then the manga because Yukimaru, Guren and the other Oto ninjas were not in the plot; as well as in the manga, Naruto and the Konoha ninjas did not go on a mission at all. One blow from Deidara's clay bomb captured the Three-Tails.

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