Shukaku

, better known as the is a tailed beast that was sealed within Gaara of Sunagakure. It has been captured by Deidara, then extracted and sealed by Akatsuki.

Background
According to Tobi, Shukaku 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. Citizens of Sunagakure believed that Shukaku was a of a corrupted Suna priest, and was originally sealed inside a tea kettle.

Shukaku had two jinchūriki in the past, one of which inspired the Third Kazekage to create the Iron Sand technique. Shukaku was eventually removed from their bodies, resulting in their deaths.

When Karura became pregnant with Gaara, her husband, the Fourth Kazekage, ordered Chiyo to seal Shukaku into Gaara during his birth. Before Karura died from a premature childbirth, she passionately told her son that she would protect him and her love seeped into the sand. However, Gaara couldn't properly control Shukaku's powers, killing or injuring several of Suna's shinobi and citizens. Deemed a failure, the Fourth Kazekage chose to test his son, ordering Yashamaru to assassinate him and also tell him that his mother never loved him, and that she was used as a human sacrifice for the sealing process. Gaara failed the test, fully releasing the rampaging Shukaku, to which the Fourth eventually used his Gold Dust to subdue the beast.

It was revealed that 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.

Personality
Shukaku is cunning, but also incoherent, giving it the demeanour of a drunkard, which fits well with the typical depictions of tanuki in Japanese folklore. According to Gaara, because of its innate rage and bloodlust, Shukaku relishes the opportunity to kill anyone in sight, even more so during a full moon. Like all tailed beasts, Shukaku also has a strong desire to preserve its own life, and as such, will grant its jinchūriki the power to control sand as well as allow them to transform into it in a state of need and bloodlust.

The Eight-Tails mentioned that Shukaku hates Kurama for its beliefs that the tailed beasts' strength is measured by the number of tails and, as such, views Shukaku as the weakest.

Appearance
Shukaku is a sand-coloured tanuki, with black (dark blue in the anime) markings all over its face, body, and tail, and a jagged, concave mouth. The sclera of its eyes are black, containing yellow irides with pupils that each takes the shape of a black four-pointed star with four black dots around it.

Abilities
Like all tailed beasts, Shukaku has an enormous supply of chakra. However, it does not have as much as Kurama. Shukaku has the power over wind, in which it uses to perform powerful attacks such as the Wind Release: Drilling Air Bullet; the ability to manipulate sand, using it as an effective weapon; and can perform the Tailed Beast Ball. At fully-released form, Shukaku's body is completely made of sand; as such he can ignore most damage that would otherwise be detrimental, as shown when he had one of his arms chopped off by Gamabunta and was still able to keep on fighting. Whenever Shukaku is sealed into its jinchūriki, it has the ability to eat away its host's personality, and influence him or her with its bloodlust behaviour whenever he or she sleeps, which leads its host to develop insomnia.

Activity inside Gaara
As Shukaku's jinchūriki, Gaara has the ability to move and levitate sand at will. Shukaku was at first believed to be the source of the automatic Shield of Sand, however the Fourth Kazekage told Gaara that this was due to his mother's will being in the sand. When angry or in a state of bloodlust, Gaara can use the sand in his gourd to take on the form of a miniature Shukaku, or even create a giant copy of Shukaku's body.

Invasion of Konoha Arc
During the Invasion of Konoha, Gaara transformed into his full Shukaku form during his fight with Naruto Uzumaki, who later summoned Gamabunta. When Gaara exposed himself on top of Shukaku's head and used the Feigning Sleep Technique, Shukaku took full control of its body and battled them. Shukaku's power was too much for Gamabunta to handle, resulting in both him and Naruto using the Combination Transformation to take the form of a giant fox (Kurama in the anime). Gamabunta used the form's claws and teeth to grab and restrain Shukaku, in order for Naruto to wake Gaara up with a punch, which suppressed Shukaku's power.

Kazekage Rescue Arc
During Gaara's time as the Fifth Kazekage, Deidara and Sasori from the Akatsuki were assigned to capture Shukaku. After Deidara defeated Gaara in combat and took him to Akatsuki's lair in the Land of Rivers, Shukaku was forcefully extracted from Gaara, thus killing him. Gaara was resurrected through a combination of Chiyo's life-transferring technique and Naruto's chakra. Even though he is no longer the host of Shukaku, Gaara is still able to control sand.

Influence
is the Japanese name for the raccoon dog. The tanuki has been renowned for its mischievous nature and abilities to shape-shift and bring good fortune. Shukaku has ties to several tanuki archetypes. One is the Bunbuku Chagama tale, where a tanuki sealed inside a tea kettle is then placed inside its first host. In the series, Shukaku was reported to have been sealed within a tea kettle. Some tales state of a shrine priest being a tanuki in disguise, which ties into how, early in the series, Shukaku was reportedly a corrupted Suna priest.

Trivia

 * Since Gaara was infused with Shukaku prior to his birth, it would seem that its abilities were melded with Gaara's own; however, it's unclear if Gaara's control of sand and the wind element is a result of his own chakra or Shukaku's.
 * Shukaku is the only tailed beast to use an actual technique; the Tailed Beast Ball, which all tailed beast can use, is less of a technique by normal standards and more of a manipulation of pure chakra.
 * It is the only tailed beast not to be resealed into its former host during the Fourth Shinobi World War. This is due to the fact that its previous host is still alive, having been revived after being killed during the extraction of the beast.
 * Previously, Shukaku was the only tailed beast with a given name. As of chapter 568, Son Gokū, the Four-Tails, revealed that all the beasts have given names from the Sage of the Six Paths.

Quotes

 * (To Naruto and Gamabunta while laughing) "I'm finally out!! And here's someone that I want to kill right now!!"