Isobu

The is a large, turtle-like tailed beast. It was formerly sealed in Yagura of Kirigakure, but was extracted for unknown reasons. It has been captured by Deidara and Tobi, then sealed by Akatsuki.

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

At some point in the past, the Three-Tails fell into the possession of Kirigakure and was sealed within Yagura, the Fourth Mizukage. It was later extracted from its host for some reason, and escaped into the wild. Because it did not have a jinchūriki, it was not intelligent enough to effectively utilize its great strength, instead being reduced to animal instincts.

The Three-Tails was depicted as having no hind-legs, with only a pair of fore-legs and its three distinctive tails. Its right eye was constantly closed, indicating some sort of injury. Because of this, it was particularly vulnerable to attacks directed at its left eye.

Abilities
Other than being a very fast swimmer despite its size, the Three-Tails' abilities were not seen in the manga. Tobi suggested that Kisame would have been a better choice for capturing the beast, given its affinity for water. In the anime, its abilities were greatly elaborated upon, many of which dealt with its size: it could roar to repel attacks; it could produce a large tidal wave that spread away from it in all directions; it could strike with any of its three spiked tails to break through most defenses. Its skin was resistant to attack in a similar fashion.

Of the beast's more specialized abilities, it could produce a hallucinogenic mist that exploited the victim's insecurities and forced the victim to face them. It could also shoot white, Menacing Ball-like shockwaves. When it swallowed something, thousands of "mini-clones" of itself would attack what it had eaten within its gut, presumably to facilitate digestion.

Three-Tailed Beast arc
In the anime, Orochimaru instructed Kabuto Yakushi and Team Guren to take a young boy named Yūkimaru to a lake. After they arrived, Kabuto forced Yūkimaru to release all of his chakra, which caused the Three-Tails to begin to surface from the lake. With Guren's help, the beast was completely revealed, and they attempted to capture it. Their attempts proved insufficient, however, and the beast created a large tidal wave to wash its attackers away.

The tidal wave gained the attention of a nearby group of Konohagakure ninja, who tried to use Four-Corner Sealing Barrier to permanently trap the beast in its own dimension. Just as the sealing began to work, the Konoha ninja were interrupted by Guren, which prompted the beast to attack them all. Yūkimaru was able to calm the beast down temporarily, but, after he became tired, the Three-Tails turned its attention to him. Guren and Naruto Uzumaki came to his defense, but were swallowed in the process. With the help of their allies on the outside, they were able to escape, and the Konoha ninja tried once more to seal the beast.

Just when the sealing was almost complete, Yūkimaru, angered by the apparent death of Guren, empowered the beast to break free of its restraints. The Three-Tails went on a rampage, attacking everyone nearby and crushing Nurari, Kigiri, and Kiho. Its attempts to attack Yūkimaru, however, had no effect. It was only through the use of the Wind Release: Toad Oil Flame Bullet that the beast was driven away. The sealing attempt was left to ANBU members, and the remaining Konoha ninja returned home. Deidara and Tobi of Akatsuki soon found these ANBU members and disposed of them.

The beast's capture took place much earlier in the manga (at the start of the Hidan and Kakuzu arc), but played out in much the same way as in the anime. Tobi and Deidara confronted the beast, to which it responded by beginning to chase Tobi. With the Three-Tails distracted, Deidara molded an explosive clay fish in an attempt to capture it. It was unclear if this marked the end of the battle, as the remainder of the battle went unseen. As the two towed the Three-Tails to the Akatsuki's lair after the battle, Tobi gloated at bringing the Three-Tails down with his special jutsu, but Deidara thought otherwise. It was later sealed in the sealing statue.

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 the 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.

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