Shukaku

or is said to be the corrupted spirit of a Sand monk turned demon. He was originally sealed inside a teakettle and then placed inside his first host, similar to the tale of Bunbuku Chagama. He was sealed inside Gaara by Chiyo on the orders of Gaara's father, the Fourth Kazekage, using Gaara's mother as the sacrifice. Shukaku had two other hosts before Gaara, both of whom died when Shukaku was extracted from them. Shukaku is intelligent but also incoherent, giving him the demeanor of a drunkard, this fits well with the typical depictions of tanuki in Japanese folklore. Shukaku itself can translate to "drunk" in Japanese. Like other tailed beasts, its innate rage and bloodlust influence its behavior, and it relishes in the opportunity to kill anyone in sight.
 * Name: One-Tailed Shukaku (一尾の守鶴 Ichibi No Shukaku) or Shukaku of the Sand (砂の守鶴 ''Suna no Shukaku)
 * Creature Type Tanuki (racoon-dog, native only to Japan and other Asian countries)
 * Host None, Formerly Gaara and previously two unknown Sunagakure citizens.
 * Unique traits: Ability to spit concentrated balls of wind mixed with chakra
 * Statues: Captured, extracted, and sealed by Akatsuki

As a host of Shukaku, Gaara has the ability to move and levitate sand at will, and is protected at all times by the Shield of Sand, regardless of his will (even self-inflicted injuries are prevented by the sand). He also suffers from lifelong insomnia, a side effect of being Shukaku's host. Should he fall asleep, Shukaku would eat away his personality making him a monster.

When angry or particularly bloodthirsty, Gaara begins to transform into a human-sized version of the Shukaku by layering sand over his body. The sand takes on the appearance of the Shukaku, and enhances his abilities greatly. While in this form, Gaara gains immense strength, endurance, and the ability to use Sand Shuriken. The weak point of this transformation is at the base of its tail, where any strong enough blow (such as an exploding note) will cause severe damage to it. He can also stretch his sand limbs as he does with his regular sand, the primary difference being that his limbs remain just as strong as when they are retracted. This transformation seems to occur very slowly, as both attempts to assume this form during the Chunin exams were foiled before completion. Gaara has only been able to reach a point at which all but his legs have been covered in sand.

In dire situations, Gaara can create a life-sized replica of Shukaku from sand. Unlike his human-sized version, this replica can be created almost instantly. Gaara controls the replica from within, remaining safe from enemy attacks. Despite being made of sand, Gaara seems to be unable to regenerate any damage caused to the replica. While in this form, Gaara can also release Shukaku's spirit, allowing the demon to use his full power. To do this, Gaara must emerge from within the replica, settling on the top of its head (possibly, to avoid suffocation from spending too long unconscious inside the sand) and force sleep upon himself. Once released, Shukaku can inhale deeply, distending its own stomach, and punch his stomach to use Wind Release: Drilling Air Bullet (風遁・練空弾 Fūton: Renkūdan?), shooting a large ball of compressed air and chakra at an opponent. The ball explodes when ever it hits something large enough (like a large tree), and packs enough power to even severely injure Gamabunta. Because Gaara protrudes from the top of Shukaku's head, he can be awoken, thus suppressing Shukaku's spirit once more.

Part II
Deidara is assigned to capture Shukaku. After defeating Gaara in combat and taking him to Akatsuki's lair, Shukaku is forcefully extracted from Gaara, thus killing him. Gaara is resurrected through a combination of Chiyo's life-transferring technique and Naruto's chakra. Gaara's ability to control sand still appears to be intact, though to what degree remains unclear.

Speculation
Since Gaara was infused with the Shukaku from birth, it would seem that the Jinchuuriki's elemental chakra abilities melded with Gaara's own, as the Kazekage used sand to pull Naruto's hand to his own in the manga. The ability the Shukaku grants is most likely wind nature and earth nature chakra. Kakashi's explanations in Part II indicate that elemental natures of variable type from normal result from the mixture of multiple natures simultaneously. Therefore, although there is no proof of this in either manga or anime, it would seem that a mixture of wind and earth natures of chakra would be sufficient to produce the sand manipulation.