Isobu

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

Background
At some point in the past, the Three-Tails was sealed within an unnamed shinobi from Kirigakure, later it seemed to have been extracted from its Jinchūriki for reasons unclear, though Tsunade stated that the Three-Tails was lost after the great war, and was set free to the wild.

The Three-Tails weak spot is its eye, the spikes covering his eye blocked a Rasengan from hitting his eye, and it keeps it's other eye closed at all times. The Three-Tails is depicted has having no hind-legs, only a pair of fore-legs and it's three distinctive tails.

Within the beast's stomach, there is an entire legion of miniature Three-Tails which are probably used for digestion of what ever the Three-Tails manages to consume, by means of launching itself clear out of the water, high into the air and then comes down with all it's weight, consuming it's unfortunate victims whole as it comes crashing down.

Three-Tails Arrival arc
During the second Shippūden filler arc, a boy named Yūkimaru has the unique ability to control the Three-Tails (or at the very least summon it). It has been speculated that Yūkimaru 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, Yūkimaru and Kabuto Yakushi traveled to the lake. When Kabuto forced Yūkimaru 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 Yūkimaru again, this time also with Guren. By having the two combine 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 it 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 Yūkimaru releasing his chakra.

After the Three-Tails is freed from Yūkimaru tiring, the beast shows enough rationality to know it needs to get rid of Yūkimaru 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-Tails' dimensional barrier. On the other side, Naruto used his Rasengan to break through the Three-Tails' body. This somehow caused Naruto and Guren to be sucked out of the Three-Tails.

Later on the Three-Tails was almost successfully sealed by the Sealing team, but Yūkimaru's anger over Guren's apparent death empowered the Three-Tails, which managed to break out of the jutsu. The Three-Tails then attacked Nurari, Kirigi, and Kiho while they were in a fused monster form, presumably killing them. It then approached Yūkimaru and tried multiple Menacing Ball attacks, none of which were able to strike Yūkimaru. Naruto then tried to attack it with shadow clones using Rasengan, but they were all defeated without effect on the Three-Tails. Naruto summoned Gamakichi and Gamatatsu to help him; they were just about to use Wind Release: Toad Gun but Gamakichi decided to use oil to create a powerful blaze, rather than water. Using both Gamakichi's and Gamatatsu's abilities Naruto formed a new collaboration ninjutsu, Wind Release: Toad Oil Flame Bullet, which drove the Three-Tails away. Yūkimaru collapsed from exhaustion shortly thereafter.

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. Given that its overall reach is very limited considering the structure of all of its limbs, the Giant Turtle's favored method of attack at close range is by striking at a target with one or all of its three trademark tails (all of which are just about as long as its main body) in a similar manner to a scorpion attempting to sting its prey. 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. So far the only weakness it has shown is its face (or his open eye), where it covers it up every time someone tries to hit it.

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 both 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 within 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 Yūkimaru, 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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