Shukaku

or is said to be the corrupted spirit of a Sand monk that turned into a demon. He was originally sealed inside a teakettle and then placed inside his first host. He was sealed inside Gaara by Chiyo on the orders of Gaara's father, the Fourth Kazekage, using Gaara's mother as the sacrifice, but only after being removed from a second host.
 * Name: or
 * 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 the Akatsuki

Personality
Shukaku is intelligent, but also incoherent, giving it 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.

Activity inside Gaara
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. Whenever he falls asleep, Shukaku would eat away his personality, making him a monster. The Shukaku, knowing full well that if his host dies, he dies too, can control sand himself to automatically protect Gaara's body from any harm.

When angry or in a state of bloodlust, Gaara can use the sand in his gourd to take the form of a miniature Shukaku or even create the giant copy of the Shukaku's body. It is only when Gaara begins to take on the Shukaku's forms is he allowed access to its wind element, such as Wind Release: Drilling Air Bullet. Gaara can even allow the Shukaku to take over the full Shukaku form by going to sleep, but is left vulnerable as his sleeping body is left unprotected on the Shukaku's head.

Extracted
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.

Trivia

 * Since Gaara was infused with the Shukaku from birth, it would seem that the jinchūriki's elemental chakra abilities melded with Gaara's own, as the Kazekage used sand to pull Naruto's hand to his own. 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.
 * He was originally sealed inside a teakettle and then placed inside his first host, similar to the tale of Bunbuku Chagama