Shukaku

The is a monster tanuki. It was sealed within Gaara of Sunagakure since his birth.

Background
According to Madara Uchiha, the One-Tails came into being in the waning days of the Sage of the Six Paths' life; to prevent its resurgence, the Sage split up the chakra of the Ten-Tailed Beast into nine separate, monstrous constructs of living chakra, which would become known as the tailed beasts. Citizens of Sunagakure believed that Shukaku, as the One-Tails came to be called, was the corrupted spirit of a Suna priest that turned into a demon. It was originally sealed inside a tea kettle, and then placed inside its first jinchūriki.

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 jinchūriki. Karura died as a sacrifice for the sealing process, and cursed all the people of Suna, including her husband, for what they had done. Apparently, one of Shukaku's former jinchūriki was the one who inspired the Third Kazekage to create the Iron Sand technique.

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 the opportunity to kill anyone in sight, even more so during a full moon, according to Gaara. Gaara knows how to control it sometimes, but most of the time it simply forces him to let it out.

Konoha Invasion 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 (Gamakichi commented "That's one funky demon") and battled them. Shukaku's power was too much for Gamabunta to handle, resulting in Naruto using the Combination Transformation to take the form of a giant fox, which allowed Gamabunta to grab Shukaku in order for Naruto to punch Gaara in order 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. Even though he is no longer the host of Shukaku, Gaara is still able to control sand.

Activity inside Gaara
As Shukaku's jinchūriki, 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 jinchūriki. Whenever he fell asleep, Shukaku would eat away his personality, turning him into a monster.

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. It is only when Gaara begins to take on Shukaku's form that he is allowed access to its wind element, such as Wind Release: Drilling Air Bullet. Gaara can even allow Shukaku to take on 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 (Shukaku, however, is a ). 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 the demon'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.