The tailed beasts are nine giant demons in the manga and anime series Naruto. Those seen thus far are slightly larger than Gamabunta, who is over 100 meters tall. The tailed beasts each have a different number of tails, and each is a different type of creature. Compared to humans, tailed beasts possess a massive amount of chakra, described in the series as the "ultimate chakra". Shukaku seems to possess a chakra level several times that of any human, while the nine-tailed fox's chakra is practically limitless. Despite their unrivaled strength and power, not all of the tailed beasts are able to use it as effectively. The three-tailed beast, for instance, did not possess the intellect to control its own strength, making it weaker.

Those within whom a tailed beast has been sealed at birth are called jinchuriki, or simply hosts, and grow up to exhibit extraordinary powers. In some instances, they are actually stronger than the tailed beast they possess. This is because they have the knowledge to control the tailed beast's strength effectively, whereas an unsealed tailed beast may lack the necessary intelligence to do so. The host also tends to have some sort of physical characteristic akin to the tailed beast sealed within them (Naruto's whisker-marks, the rings around Gaara's eyes, and Yugito's slanted eyes). Naruto takes the word jinchuriki offensively, regarding it as a dehumanizing term. According to Akatsuki members, those who host such creatures are typically lonely people who loathe humanity, and so far this has proven to be true (at least at first). When the first two were captured, no one from their villages cared that they were taken or lifted a finger to help; in fact, they were thankful to be rid of them. It is known that the death of a host will result in the death of the tailed beast within, and the hosts seen thus far sometimes exhibit involuntary defensive powers. Likewise, the removal of the tailed beast results in the death of its host.

During the Great Shinobi Wars, each country's Hidden Village tried to make use of the tailed beasts for military purposes, and competed for them. However, no one was ever able to truly bring the tailed beasts under control. The practice of sealing tailed beasts within humans to harness their powers proved the only effective method of harnessing the awesome powers of the beasts.

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