Isobu

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
According to Tobi, 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.

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 it's shell, it has red, muscle-like tissue. It has a pair of human-like arms and hands, but no hind-legs. It's lower jaw is rather big and has teeth-like horns, together with it's 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. It's 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 jinchuriki host, it wasn't intelligent enough to control its own strength. 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.

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.