Isobu

, better known as the is a tailed beast that was formerly sealed in Yagura of Kirigakure. It has been captured by Deidara and Tobi, then sealed by Akatsuki. It was later resealed into a resurrected Yagura during the Fourth Shinobi World War.

Background
The Three-Tails first came into being in the waning days of the Sage of the Six Paths' life; to prevent the Ten-Tails' resurgence, the Sage split up its chakra and used his ability to create nine separate, monstrous constructs of living chakra, which would become known as the tailed beasts. Some time after being created, the Sage of the Six Paths sat down with all the tailed beasts and told them that they would always be together even when separated and that one day they would become one entity again with different names as well as forms than they did then when the time came for them to understand what true power was.

At some point in the past, the Three-Tails fell into the possession of Kirigakure and was sealed within Yagura, the Fourth Mizukage. However when Akatsuki went after it the beast was in the wild; the story behind how it came to be like this is still unknown. It was also stated by Tsunade in the anime that the Three-Tails disappeared after a great war.

Appearance
The Three-Tails primarily resembles a large turtle, but with a crab-like shell, and three shrimp-like tails. Under its shell, it has red, muscle-like tissue. It has a pair of human-like arms and hands, but no hind-legs. Its lower jaw is rather big and has teeth-like horns, together with its big forehead, which also has horns, it somewhat resembles a mouth with big teeth, making it look like the rest of the face is inside the mouth. Its eyes are dark and have red pupils. In the anime, the eyes are red with green pupils. Its right eye is constantly closed, indicating some sort of injury, and because of this, it is particularly vulnerable to attacks directed at its right eye.

Abilities
The Three-Tails has the ability to create coral, and swim at very high speeds. Like all Tailed Beasts, it has an enormous amount of chakra compared to most ninja and can form the Tailed Beasts' ultimate attack, the Tailed Beast Ball. Tobi suggested that Kisame would have been a better choice for capturing the beast, given its affinity for water. Deidara claimed that because the Three-Tails lacked a host, it couldn't control its own strength. The Three-Tails has also been shown to be able to roll into a ball and attack its opponents, making full use of its shell and its many protrusions in a manner similar to the Akimichi clan's Spiked Human Bullet Tank.

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 defences. Its skin was resistant to attack in a similar fashion. With these abilities, it overwhelmed Guren's Crystal Release several times over the course of their battle.

Of the beast's more specialised abilities, it could produce a hallucinogenic mist that exploited the victim's insecurities and forced the victim to face them. It could also shoot two powerful chakra infused water balls at the same time. When it ate something, thousands of miniature versions of itself would attack what it had eaten within its stomach, presumably to speed up the digestion process.

Three-Tails 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 defence, 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 Kihō. 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 moulded 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 technique, but Deidara thought otherwise. It was later sealed in the sealing statue.

Shinobi World War Arc
After Son Gokū is resealed into the Demonic Statue of the Outer Path, Tobi, who is intent on going all-out from this point on, forces Yagura to fully transform into the Three-Tails.

Preparing to attack the shinobi, the Three-Tails along with the other four beasts create Tailed Beast Balls. These attacks however, are deflected by Naruto and they are rebuffed by a now fully-transformed Naruto. Wasting no time in their attacks, the Three-Tails evades Kurama's attack and then, rolling into a ball, attempts to attack the beast while the Six-Tails had incapacitated him. It is however stopped by the Eight-Tails. Freeing itself from the beast's grasp, it and the other opposing beasts prepare a collaborative Tailed Beast Ball which is met by one from Naruto and the Nine-Tails.

As Naruto enters a deeper plain of the tailed beasts' consciousness, Isopū summons Naruto before it, telling him to stretch out his hand and that it would fulfil the promise it had made to Son Gokū. Later as the beast is freed from the chakra receiver it affirms Kurama's belief that Naruto was the person the Sage of the Six Paths had spoken about so long ago before being resealed into the Demonic Statue of the Outer Path.

Influence
Turtles are often used as the base design for sea monsters in 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.