Tailed Beast

The are the nine 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.

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
Long ago before the founding of the shinobi villages, during an era of endless wars between humans, there existed the Shinju, a tree that was revered as a deity which bore a fruit once every thousand years. Although it was forbidden to consume the Shinju's fruit, it was eaten by Princess Kaguya Ōtsutsuki, who used the chakra infused into her body to end the wars plaguing the lands. However, after Kaguya gave birth to a son who inherited the same type of powerful chakra, the Shinju's desire to reclaim its stolen power caused the tree to assume a monstrous form of what many christened as the Ten-Tails. The furious Ten-Tails rampaged throughout the lands until Kaguya's son managed to single-handedly defeat the beast and sealed it within himself. This act, along with many other great deeds in his life, resulted with Hagoromo becoming revered as the Sage of the Six Paths.

However, knowing that his death would unleash the Ten-Tails back into the world to continue its rampage and reclaim the chakra that mankind now possesses, the Sage separated the monster's chakra from its body before using Chibaku Tensei to seal the husk in what would become the moon. From there, the Sage used his Creation of All Things ability to divide the chakra into nine fragments that are each given a physical form and name. Some time after their creation, the young tailed beasts were told that they were all still linked to one another despite being separate entities. The Sage also told them that would eventually become one again, yet not as they originally were, and that one shall appear at that time to show them what true power is. Despite their close relation to one another, there is a significant amount of friction between the tailed beasts that caused them to part ways. One such cause was due to Kurama's belief that their strength was determined by the number of their tails.

Over the centuries, countless humans failed to see the tailed beasts as sapient individuals with their own personalities and emotions, and because of their immense power, they sought the tailed beasts out as mere tools to be used or taken, a sentiment they absolutely detest. By the time of the formation of the shinobi villages, after his victory against Madara Uchiha in the Valley of the End, 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, 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.

Madara, later revealed to have survived the altercation, had 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. Irked by the slow progress of the plan, assuming the identity of Tobi to oversee the matters and converting reincarnated hosts into his personalised Six Paths of Pain, Obito 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, Obito 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 matured state.

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 beasts sealed within the Ten-Tails once it was weakened enough. After Obito became its jinchūriki, the Ten-Tails assumes its true form to enact its host's desire while draining the chakra of those can it grasps in its branches.

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

 * 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.
 * 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.
 * 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.
 * Shukaku is the only tailed beast shown to have control over a physical property rather than using a chakra-based element, and hasn't shown the characteristic red-chakra-shrouded Version 1 or Version 2 forms.
 * Among the tailed beasts, only Kurama has been shown to be subject to the Summoning Technique.
 * A tailed beast can "die" with its jinchūriki, if it is still sealed within them at the time of its host's death, but the beast itself will be revived later.
 * Since Rin Nohara died without having Isobu extracted, Isobu is the first known tailed beast to have "died" and revived.
 * 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, the six tailed beasts that were used as Obito's Six Paths of Pain, all shared a voice actor with one of each other's respective jinchūriki.