Tailed Beast

The are the ten titanic behemoths within the Naruto series. They are differentiated by the number of tails they have, ranging from one to nine. The tailed beasts are living forms of chakra, sometimes referred to as, giving them immense reserves of especially strong chakra that far outmatches most shinobi. The tailed beasts' chakra is coloured red.

According to some, the immense strength that the tailed beasts possess is tempered by their bestial nature, which restricts them from being intelligent enough to use it effectively, thus 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. However, this was proven to be false, as the tailed beasts themselves are in fact sapient individuals with their own personalities and emotions, and because of their power, they were sought out by countless humans throughout the ages, and were usually treated as mere tools to be used or taken, a sentiment they absolutely detest. In addition, they are well aware of their origin and the state of the ninja world.

They also have the ability to communicate telepathically with each other and their jinchūriki, and possess several levels of consciousness. There is the initial level where the beast may appear in a habitat of sorts seemingly imprisoned in some way by whatever fūinjutsu that sealed them. The second level is where all the tailed beasts can convene, free of any restrictions and communicate with one another. The jinchūriki are also able to enter there, but in order to do so, they must be completely linked with their tailed beasts.

The tailed beasts are also able to freely transfer their chakra to whomever they choose, and as Kurama has noted, is a less complicated process for them than it is for humans.

History
In the beginning of ninja history, centuries before the founding of the ninja villages, there existed a god-like entity and original tailed beast known as the Ten-Tails, which 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 tailed beasts, before using Chibaku Tensei to seal its body in what would become the moon. Some time after being created, the Sage gave each of the young tailed beasts a name and told them that they would always be together, even when separated. He also told them that one day, they would become one entity again with a different name and form when the one would came to show them what true power is. Despite their close relation to one another, there is a significant amount of friction between them. One such cause was due to Kurama's belief that their strength was determined by their number of tails.

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 the beasts 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.

With the power of the Sharingan, Madara Uchiha was able to control Kurama and use it during his battle with Hashirama Senju. During the battle, however, control of the beast was wrested from him, and he was thought to be defeated at the Valley of the End. However it was later revealed that he had in fact survived the altercation, and obtained Hashirama's DNA — his true purpose for the encounter. Integrating Hashirama's DNA into his wounds, Madara awakened the Rinnegan shortly before the end of his natural lifespan. With it, he was able to summon the vessel of the Ten-Tails from the moon. However, using artificial means to expand his life, Madara was unable to carry out his plan to revive the Ten-Tails. As such he passed on this task to a young Obito Uchiha who had come to share his view on the world after Rin Nohara's death.

With the help of Akatsuki, by the time of the Fourth Shinobi World War, seven of the nine tailed beasts had been sealed into the statue. Later, all the beasts — with the exception of Shukaku — were resealed into their respective, reincarnated hosts which had been made into a personalised Six Paths of Pain for Tobi. Irked by the slow progress of the plan, Tobi accompanied by the statue entered the battlefield to obtain the Benihisago and Kohaku no Jōhei which contained the Gold and Silver Brothers. Despite not having sealed the last remaining two tailed beasts entirely — only possessing portions of their power, Tobi decided to begin the plan as the Demonic Statue of the Outer Path undergoes a painful regeneration into the Ten-Tails. When Madara realised the situation, he attempted to rectify this. Despite Naruto and Killer B's attempt to destroy the Demonic Statue while in mid-transformation, it was successfully restored into the Ten-Tails, albeit in an incomplete form for a while before reaching its full form.

However, the tailed beasts used in the Ten-Tails' recreation transfered bits of their chakra into Naruto prior to Obito carrying out the process as the Ten-Tails sees them supporting the human in spirit as they all have recognise him as the youth that the Sage of the Six Paths foretold. Naruto and Kurama intend to release the tailed beast sealed within the Ten-Tails once it was weakened enough.

The Tailed Beasts
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Similar Cases

 * 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.
 * In Naruto 6: Road to Ninja, Kurama has a Genjutsu World counterpart called the, who is sealed within the body of Naruto's alternate self, Menma. Menma creates and controls living entities composed of its chakra known as the Nine Masked Beasts.
 * In Naruto Shippūden: Dragon Blade Chronicles, there are five dragons known as the Genryū, each possessing one of the five chakra natures. If all the Genryū are combined, they become the Light Dragon.
 * In Naruto Shippūden: Kizuna Drive, there is the Three-Headed Guardian Beast, guardian of Yumegakure which, like the tailed beasts, is a living form of chakra.
 * In Naruto Shippūden: Ninja Destiny 3, there are three featured in the game, which like the other tailed beasts (that have been featured in the game) can be fought via Beast Battle Mode.

Trivia

 * Despite all the tailed beasts gaining a name from the Sage of the Six Paths, only some such as Son Gokū refer to themselves by that name on a regular basis. Others, such as Kurama and Gyūki, prefer to refer to each other by the number of tails each possess.
 * Shukaku is the only tailed beast shown to have control over a physical property rather than using a chakra-based element.
 * Because of the moon's origin and it being the holder of the Ten-Tails' body, tailed beast and subsequently their jinchūriki are effected by it, and much more so during a full moon.
 * According to Gaara, Shukaku relishes the opportunity to kill anyone in sight at any time because of its innate rage and blood-lust, but especially so during the full moon.
 * In the anime, Fukai, during his time as Gyūki's jinchūriki, noted that he would have trouble sleeping at night, especially during a full moon and at one point was shown to have to suppress a partial manifestation of Gyūki's tail after he had been staring at the full moon. Fukai was also shown to take sleep pills to help with this side-effect of being a jinchūriki.
 * Several non-jinchūriki shinobi have been said to hold chakra levels, reserves or strength, comparable to that of a tailed beast. Such individuals include Kisame Hoshigaki, Hashirama Senju, Madara Uchiha, the Third Raikage, and A. In fact, the former has been given the moniker of . In addition, Madara has claimed that the power of his final Susanoo is comparable to that of a tailed beast.
 * Among the tailed beasts, only Kurama has been shown to be subject to the Summoning Technique.
 * 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 Gyūki, the group has only six, the seventh missing beast being Saiken to make room for an additional anime-only arc.
 * In the anime, excluding Shukaku, Gyūki and Kurama, the tailed beasts share voice actors of the other jinchūriki.