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-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 Three-Tails lived in a lake and remained uncaptured by any shinobi. According to Tsunade, the Three-Tails was lost after the great war. It's eye is its weak spot as the spikes covering his eye blocked a Rasengan from hitting his eye. Inside the beast's stomach, there are a lot of minature Three-Tails which are probably used for digestion.

Three-Tails Arrival filler arc
During the second Shippūden filler arc, a boy named Yukimaru has the unique ability to control the Three-Tails (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 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-Tails 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 Three-Tails' 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. Later on when Shizune, Sakura, Ino and Hinata are performing the Four Corner Sealing Barrier it is seen that his legs were caught in the chakra in the net, however interference from Guren caused the seal to be broken and the the three-tails to be set loose. It proceeded to use an attack similar in nature to Naruto's Four-Tailed Fox Menacing Ball, although the gathering of chakra had a white color and caused a shockwave upon release, instead of traveling along as its own attack. The Three-Tails began to loom over Guren, who had been knocked unconcious because of the attack, but was soon interrupted by Yukimaru releasing his chakra.

After the Three-Tails is freed from Yukimaru tiring, the beast shows enough rationality to know it needs to get rid of Yukimaru for its freedom. After a heated battle against Guren and Naruto, the beast manages to swallow them whole.

Inside the beast, several miniature copies (possibly enzymes to break down "food") began attacking Naruto and Guren relentlessly. Before they could succeed, Kakashi and Yamato were able to break through the Three-Tail's dimensional barrier, this somehow caused Naruto and Guren to be sucked out of the Three-Tails.

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 Three-Tails 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 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 manga. The Three-Tails is clearly quite fast despite it's massive size, as evidenced by its ability to keep up with Tobi, who was running on the water at the time. While fighting the Three-Tails, Tobi complained that Kisame would've been a better choice to capture the creature because of the Three-Tails' affinity with the water, which suggests that it could use water techniques, despite not being demonstrated.

In the anime, however, the Three-Tails' abilities have been elaborated upon. It's skin is shown to be very hard, that is able to shatter Guren's Crystal Release techniques with ease. It can 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. It can also perform a 360° tidal wave technique that can cause large scale damage in all directions. Finally, the Three-Tails has been shown to possess limited genjutsu capabilities too, as it can create a hallucinating mist around a certain area of its choice, that can confuse both people and animals around with hallucinations. Guren deduced that the mist exploits the victim's insecurities and forces the victim to face them.

One other ability that the Three-Tails has been shown to use is the opening of an inter-dimensional portal within the lake that it appears from in the anime. Whether or not this ability is exclusive to the Three-Tails or if the other Tailed Beasts can use it as well is unknown. It has also not been revealed if the Tailed Beasts reside within their own dimensions together or separated.

After the Three-Tails swallows Naruto and Guren, its internal body structure is also revealed to be very hard and impenetrable, much like its outer skin. Its stomach is enormous, and is studded with numerous spike-like structures. Also, the Three-Tails houses thousands of "mini-clones" of itself in its gut, that attack its swallowed prey to presumably destroy and digest it.

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. Sometime after this filler arc, Deidara captured the Three-Tails with the use of his explosive clay techniques.
 * Another difference is that in the manga version, Deidara and Tobi fought the Three-Tails during the Hidan and Kakuzu arc while in the anime version, the Three-Tails appears directly after.
 * The face of the Three-Tails resembles the face of an one-eyed man with masked jaws, furthermore, it is shown to even have a nose much resembling of a human's. It was more detailed in the anime.
 * A more of a imagery difference is that the manga colored version of the beast depicts it being a sea green color and red. In the anime filler arc, it is shown to be grey and red, respectively.

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