Tailed Beast

The are nine giant demons that serve to drive the plot of the Naruto series. They are differentiated by the number of tails they have, with each possessing a number from one to nine. The tailed beasts are large, living forms of chakra, sometimes referred to as, giving them power that far outmatches most shinobi. However, this immense strength is tempered due to their bestial nature, which restricts them from being intelligent enough to use it effectively. When the tailed beasts are sealed within humans, they are able to benefit from the knowledge of their hosts, giving them the ability to speak and better utilize their powers. Akatsuki has been capturing the tailed beasts and their hosts, and as of the Fourth Great Shinobi World War possess seven of the nine beasts.

History
In the beginning of ninja history, centuries before the founding of the ninja villages, there existed one demon, the Ten-Tailed Beast, that plagued the planet until the Sage of the Six Paths defeated and sealed the beast within himself, making him the first jinchūriki. On his deathbed, he divided the Ten-Tails' chakra into the nine other tailed beasts and sealed its body in the moon.

By the time of the formation of the shinobi villages, Hashirama Senju, the First Hokage, managed to capture and control some of them with his Wood Release techniques, and used them as peace treaties to the other great shinobi nations in order to stabilize the balance of power between them. After his death, however, the shinobi nations couldn't control these giant demons themselves and they began to run wild, so the nations instead moved to seal the tailed beasts within humans. In order to ensure that the jinchūriki wouldn't turn traitor, it has been common practice to choose the jinchūriki from the village's Kage family, such as a sibling or a spouse. The strength of the jinchūriki would then be used to protect the village and be a living display of the Kage's power.

Comments by both the Eight-Tails and the Nine-Tails indicates that those two at least know each other on a personal level, though without much respect. Moreover, the Nine-Tails' reaction to Naruto's new seal indicated that it also had knowledge of the Sage of the Six Paths, and by extension the Ten-Tails.

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Trivia

 * Only four people are said to have full control of the tailed beasts. They are Madara Uchiha (through his Eternal Mangekyō Sharingan), Hashirama Senju (through his special Wood Release technique), and two jinchūriki: Yagura and Killer Bee.
 * Several non-jinchūriki shinobi, such as Kisame Hoshigaki and the Fourth Raikage, have been said to hold chakra levels comparable to that of a tailed beast.
 * In the manga, before Taka began working with Akatsuki, Akatsuki was said to have seven tailed beasts. In the anime, when Taka is sent to capture the Eight-Tails, the group has only six, the seventh missing beast being the Six-Tailed Slug.
 * In Naruto Shippūden 2: Bonds, it features a creature called the Zero-Tailed Masked Leech. Despite having a numerical tail name, it is not a real tailed beast and it has characteristics that are quite different from the true tailed beasts.