Tailed Beast

The are nine giant monsters that serve to drive the plot of the Naruto series. They are differentiated by the number of tails they have, ranging 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 by their bestial nature, which restricts them from being intelligent enough to use it effectively. As such, when the tailed beasts are sealed within humans, they are able to benefit from the knowledge of their hosts, giving them the ability to better utilise their powers.

Due to their strength, the beasts are usually treated as mere tools to be used or taken, a sentiment they detest; although they have knowledge of the Sage of the Six Paths and seem to be aware of their origin as pieces of the Ten-Tails, they view themselves as individuals with their own personalities and emotions, and a good portion of their rage stems from being treated as mere "pets" of the shinobi. However, their relationships with each other are little better. Although they are able to communicate telepathically with each other and their jinchūriki, and know each other on a personal level, there is a significant amount of conflict between them. One reason is due to Kurama's belief that their strength is determined by their number of tails.

History
In the beginning of ninja history, centuries before the founding of the ninja villages, there existed one demon, the Ten-Tails, that plagued the world until the Sage of the Six Paths defeated and sealed the beast within himself, making him the first jinchūriki. However, knowing that his death would release the Ten-Tails back into the world, the Sage used his ability, the "Creation of All Things", to divide its chakra into the nine other tailed beasts, before using Chibaku Tensei to seal its body away in what would become the moon. The Sage gave each of the beasts a name.

By the time of the formation of the shinobi villages, Hashirama Senju, the First Hokage, used his Wood Release techniques to capture and give several tailed beasts to the other shinobi villages as peace treaties, in order to stabilise the balance of power between them. However, after Hashirama's death, the villages had trouble containing these giant demons themselves and they began to run wild, so the villages 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. Throughout history, very few people have ever gained full control over a tailed beast. Despite this, some have been able to use the power of the tailed beasts outside of actually becoming true jinchūriki. The legendary Gold and Silver Brothers became Kurama's pseudo-jinchūriki, as they were once devoured by it, but managed to survive by consuming some of its own normally-deadly flesh, becoming tinged with its chakra. Madara Uchiha and Tobi have both controlled Kurama with the Sharingan, even subjecting the beast to the Summoning Technique.

Akatsuki has been capturing the tailed beasts and their jinchūriki, and as of the Fourth Shinobi World War, possesses seven of the nine tailed beasts.

The Tailed Beasts
{{#arraymap:Shukaku, Two-Tails, Three-Tails, Son Goku, Five-Tails, Six-Tails, Seven-Tails, Eight-Tails, Kurama, Ten-Tails|,|!|

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Trivia

 * 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-Tails to make room for an additional anime-only arc.
 * 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 fact, the former has been given the moniker of.
 * In Naruto Shippūden 2: Bonds, it features a creature called the Zero-Tails. Despite having a numerical tail name, it is not an actual tailed beast and it has characteristics that are quite different from them.
 * Among the tailed beasts, only Kurama has been shown to be subject to the Summoning Technique.
 * The concept of a pseudo-jinchūriki was explored in an anime-exclusive arc, in which Sora was revealed to have had residual chakra from Kurama, collected after it rampaged Konoha, sealed into him. This happened before the introduction of the concept in the manga.
 * A Kumogakure ninja once attempted to replicate the Gold and Silver Brothers stunt with tentacle meat from the Eight-Tails, but died instead. The brothers' success was attributed to a possible distant relationship with the Sage of the Six Paths.
 * In the anime, the Four Celestial Symbols Men were also able to siphon Shukaku's power in order to revive their leader, Seimei.
 * In chapter 568 it's revealed that the Tailed Beast have proper names given to them by the Sage of Six Paths.