Shukaku

The is a  sealed within Gaara of Sunagakure.

Background
Shukaku was once said to be a corrupted spirit of a Sand priest that turned into a demon, but it is actually one of the Tailed Beasts. It was originally sealed inside a tea kettle and then placed inside its first host. When Karura, the wife of Fourth Kazekage, became pregnant, her husband ordered Chiyo to seal Shukaku inside an unborn Gaara, but only after being removed from the second host. Karura died as a sacrifice for the sealing process and cursed all the people of Sunagakure, including her husband, for what they had done.

Personality
Shukaku is intelligent, but also incoherent, giving it the demeanor of a drunkard, which 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. Gaara knows how to control it sometimes but most of the time it just forces him to let it all out.

Invasion of Konoha arc
During the attack on 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 (Gamakichi proclaimed that its voice was kinda funky) and battled them. Shukaku's power was too much for Gamabunta to handle; so Naruto used the Combination Transformation to take form of a giant fox, which allowed Gamabunta to grab Shukaku in order for Naruto to punch Gaara to wake him up, which suppressed Shukaku's power.

Rescue Gaara arc
During Gaara's time as the Fifth Kazekage, Deidara and Sasori from the organization 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. Gaara's ability to control sand still appears to be intact, though to what degree remains unclear.

Activity inside Gaara
As Shukaku's host, Gaara has the ability to move and levitate sand at will. In addition, Shukaku protects him at all times with 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 fell asleep, Shukaku would eat away his personality, making him a monster.

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 Shukaku's body. It is only when Gaara begins to take on Shukaku's forms that is he allowed access to its wind element, such as Wind Release: Drilling Air Bullet. Gaara can even allow Shukaku to take over the full Shukaku form by going asleep, but is left vulnerable as his sleeping body is left unprotected on Shukaku's head.

Influence
is the Japanese name for the Raccoon Dog. The tanuki has been renowned for its mischievous nature and ability 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, then placed inside its first host. In the series, Shukaku was reported to have been sealed within one. 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 Sand priest.

Trivia

 * Since Gaara was infused with Shukaku prior to his birth, it would seem that the Jinchūriki's abilities 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 known Tailed Beast with an actual name.