Tailed Beast

In the Naruto universe there are at least nine "great" demons,called the tailed beasts. An evil organization, the Akatsuki, has a sinister plan that involves the capture of the tailed beasts.

History
It seems that the Demons (also referred to as bijuu) were around before the First Great Ninja War. The shinobi nations first involved these demons by attempting to capture them and harnessing their powers to defeat their enemies. It never worked out. A technique that is used to seal demons and use their power became available (description stated above), and the demons slowly became victims of this technique. Eventually, at the time period of the Naruto plot, very few demons roamed free in the world.

Jinchuuriki
The term jinchuuriki is first used by the Akatsuki to mean the shinobi that have a demon sealed within him or her. Naruto considers this term profane and derogatory. When a newborn baby has a demon sealed inside his or her body, the person develops signs of containing the bijuu. For example: Naruto has larger than normal canine teeth and stripes on his cheeks since his sealed demon is the Fox Demon. Gaara has dark lines outlining his eyes and his gourd resembles the pack that a tanuki carries around.

One-Tailed Demon

 * Name: Ichibi no Shukaku


 * Host: Gaara (former)


 * Appearance: A tan, one-tailed tanuki made of sand


 * Abilities: Controls sand. Spits wind full of chakra. Captured by Deidara of Akatsuki thus killing Gaara. Gaara was brought back to life but for unknown reasons he can still control sand.

Two-Tailed Demon

 * Name: Nibi no Nekomata


 * Host: Yugito


 * Appearance: A orange, two-tailed cat made of fire


 * Notes: Abilities unknown, possibly linked to fire element. Captured by Hidan and Kakuzu of Akatsuki.

Three-Tailed Demon

 * Name: Sanbi


 * Host: None


 * Appearance: A blue, three-tailed turtle-like creature.

Note: A large turtle-like creature. When found by Tobi, it was not sealed within a Jinchuuriki. Though its power was unrestrained, the demon itself wasn't intelligent enough to control its own power. Captured by Deidara and Tobi of Akatsuki.

Four-Tailed Demon

 * Name: Yonbi**


 * Host: Unknown Old Man


 * Appearance: Unknown


 * Note: Able to use multiple elemental fusions.

Five-Tailed Demon

 * Name: Gobi**


 * Host: Unknown


 * Appearance: Unknown


 * Note: Unknown

Six-Tailed Demon

 * Name: Rokubi**


 * Host: Unknown


 * Appearance: Unknown


 * Note: Unknown

Seven-Tailed Demon

 * Name: Shichibi**


 * Host: Unknown


 * Appearance: Unknown


 * Note: Unknown

Eight-Tailed Demon

 * Name: Hachibi**


 * Host: Unknown


 * Appearance: Unknown


 * Note: Unknown

Nine-Tailed Fox Demon

 * Name: Kyuubi no Yoko


 * Host: Naruto


 * Appearance: A orange, nine-tailed fox


 * Abilities: Nearly unlimited chakra, near instant healing rate


 * Note: By far the most powerful bijuu. According to Japanese myth, its power is unmeasurably great. No bijuu has managed to defeat the Kyuubi in combat, even when allied against it.


 * ** denotes a demon that may or may not have been captured. Besides the first four, there are three others that Akatsuki has captured, but we don't know which ones.