The tailed beasts (尾獣, bijū), sometimes referred to as "Chakra Monsters" (チャクラのバケモノ, Chakura no Bakemono), are the nine titanic living forms of chakra created by the Sage of Six Paths out of the chakra from the Ten-Tails. They are differentiated by the number of tails they have, ranging from one to nine.
Long before the founding of the hidden villages, the Sage of Six Paths saved the world by sealing the Ten-Tails into his body, becoming its jinchūriki. Some time later, the Sage was inspired by his youngest son, Asura, to use his Creation of All Things ability to divide the Ten-Tails' chakra into nine living entities, what would become the tailed beasts. The Sage named the nine and explained to them that a link existed between them despite their separation. He also foretold of a day when they would be brought together again — though not as they were — and that at that time someone would emerge to show them what true power is. In the anime, it was shown that before his death, the Sage and his followers built temples in nine different regions of the world where the tailed beasts could live and be protected. The tailed beasts were originally created not only to prevent the Ten-Tails' resurrection, but also to maintain and balance peace. The first person to come close to their original usage was Hashirama Senju, who had captured several of the tailed beasts with his Wood Release, distributed the beasts amongst the new villages in order to stabilise the balance of power between them. However, Hashirama's usage of the tailed beasts were forceful rather than earning their wilful cooperation. This method would be realised in Hashirama's successor as Asura's reincarnation, Naruto Uzumaki.
Over the centuries, humanity failed to recognise the tailed beasts as sentient individuals, instead seeing them only as monsters, demons, or mindless beasts worthy of fear and disdain. Because of their immense power, the tailed beasts were sought out by humans to be used as weapons in times of war. The beasts resented this treatment and came to hate humans, at times willingly becoming the monsters they were viewed as. In order to harness the beasts' power, the villages began sealing them into humans, creating their own jinchūriki.
Decades later, after the Third Shinobi World War, an organisation known as Akatsuki begins capturing the villages' jinchūriki in order to extract their tailed beasts and seal them into the Demonic Statue of the Outer Path for the purposes of reviving the Ten-Tails. By the time of the Fourth Shinobi World War, Akatsuki has captured the first seven tailed beasts. In order to capture the last two, the Eight-Tails and Nine-Tails, Tobi seals the beasts back into their reincarnated jinchūriki and forces them to fight the Eight-Tails' and Nine-Tails' jinchūriki: Killer B and Naruto Uzumaki, respectively. From Naruto's determination to free them from Tobi and his desire to learn their names, the tailed beasts collectively decide that Naruto is the person the Sage of Six Paths foretold of. They give him portions of their chakra, enabling him to release them from Tobi's control.
Tobi is able to reclaim the freed tailed beasts and, using fragments of the Eight-Tails and Nine-Tails' chakra, to prematurely revive the Ten-Tails. Tobi then becomes the Ten-Tails' jinchūriki, but he is defeated by the Allied Shinobi Forces and the tailed beasts are removed from his body. The nine tailed beasts join forces against Tobi's co-conspirator, Madara Uchiha, but he quickly defeats them all and seals them, including the Eight-Tails and Nine-Tails, into the Demonic Statue and reforms the Ten-Tails yet again. Madara becomes the Ten-Tails' jinchūriki and attempts to use Infinite Tsukuyomi on the world. Naruto uses the tailed beasts' earlier donations of chakra to try and defeat him, but is unable to do so in time. Madara is afterwards unwittingly used to restore Kaguya Ōtsutsuki, who tries to take what portions of the tailed beasts' chakra Naruto still has.
In one of Kaguya's dimensions, Naruto and Sasuke Uchiha seal Kaguya away and the tailed beasts are extracted from her body, ending the war. Sasuke, however, realising that the struggle for the tailed beasts started the war in the first place, imprisons them with Chibaku Tensei and intends to kill them so the world can't fight over them any longer. Naruto convinces him not to in the end and the tailed beasts are finally released for good. The tailed beasts spread out through the world, free to live as they want; both Gyūki and Kurama decide to return and stay with B and Naruto. However, though they are apart, the tailed beasts' chakra fragments remain within Naruto, allowing his body to act as their "meeting ground".
Fifteen years after the Fourth Shinobi World War, during the investigation of the Ōtsutsuki clan to find out Kaguya's final fate, two of them began to chase the tailed beasts in order to claim their chakra, starting with Gyūki and Kurama. While Momoshiki Ōtsutsuki and Kinshiki Ōtsutsuki were initially successful in steal an amount of the powers of the respective beasts by capturing their jinchūriki, both ended up being killed by being dominated by the combined power and effort of the Five Kage, Sasuke and Boruto Uzumaki.
Sometime after the defeat of Momoshiki and Kinshiki, when Urashiki Ōtsutsuki began targeting Shukaku for its chakra, it was deduced he was also interested in all the tailed beasts' chakra. Naruto then organised a telepathic meeting with the other tailed beasts, except for Matatabi, Kokuō, and Chōmei, who each refused to accept aid from humans in fear of being captured and used again by them, despite their trust in Naruto, and thus decided to skip the meeting. With the final defeat of Urashiki, the beasts that were being kept safe presumably returned to be free to go their own way.
Tailed beasts are widely believed to be bestial in nature, incapable of the intelligence needed to fully utilise their immense strength. It is thought that only when sealed within humans are they able to benefit from the knowledge of their jinchūriki to use their powers effectively. This is not actually true. However, the Ten-Tails does tend to make wanton use of its powers, not focusing its abilities towards whatever end would be most advantageous to it until sealed into a jinchūriki.
Each of the tailed beasts have their own unique abilities, but they all share the same attributes: they have massive quantities of powerful chakra — coloured red — far greater than what most shinobi have, can easily transfer their chakra to whomever they choose, and can use the Tailed Beast Ball. Because the tailed beasts are pure chakra, they cannot actually be killed; if they or their jinchūriki die, their chakra will re-coalesce in time. In addition, if a large portion of a tailed beast's chakra is separated from it, that chakra becomes a separate, sentient copy of the tailed beast. It is also pointed out that consuming the flesh of a tailed beast is usually deadly for anyone, unless the person is somehow descended from the Sage of Six Paths; in this case, the person will gain abilities similar to a jinchūriki.
Tailed beasts can communicate telepathically with one another and their jinchūriki, and possess several levels of consciousness. In the initial level, the beast appears to be in a habitat of sorts, typically imprisoned in some way by whatever fūinjutsu was used to seal them. The second level is where all the tailed beasts can convene and communicate with one another, free of any restrictions. Jinchūriki are also able to enter this level, but in order to do so, they must be completely linked with their tailed beasts. A freed tailed beast can continue communication with their former jinchūriki, even years after separating.
Although only shown with Kurama, a tailed beast will become unconscious if they have lost all of their chakra, by either using it, or having it drained from them.
It has been shown that Tailed Beasts can have more than one conscience or body, as in the Fourth Great Ninja War and onwards, Shukaku to Chōmei are present in the real-world, and Gyūki was inside Killer Bee, while all 8 were inside Naruto as well, where they hold meetings.
The Tailed Beasts
The tailed beasts in order of their number of tails:
- In Naruto Shippūden the Movie: Bonds, the Zero-Tails, despite having a numerical tail name and being referred to as a tailed beast in the movie, it is not an actual tailed beast as it has characteristics that are quite different from them.
- In Naruto the Movie: Blood Prison, there is a yōkai called Satori that resides within the Box of Ultimate Bliss.
- In Road to Ninja: Naruto the Movie, Kurama has a Genjutsu World counterpart called the Black Nine-Tails, who is sealed within the body of Naruto's alternate self, Menma Uzumaki. Menma creates and controls living entities composed of its chakra known as the Nine Masked Beasts.
- In Naruto Jinraiden: The Day the Wolf Howled, there is a legendary beast named Rōen. Though similar in size and power to a tailed beast, Rōen is merely an animal, driven by instinct instead of intellect.
- In Naruto Shippūden: Ninja Destiny 3, there are three masked beasts (仮面獣, kamenjū) featured in the game (like Matatabi and Isobu that have been featured in the game) that can be fought via Beast Battle Mode.
- In Naruto Shippūden: Dragon Blade Chronicles, there are five giant dragons known as the Genryū, each possessing one of the five chakra natures. If all the Genryū are combined, they become the Light Genryū.
- 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: Ultimate Ninja Storm Revolution (and two anime only episodes), there is a Mecha-Kurama, a mechanised version of Kurama.
- In Naruto: Uzumaki Chronicles 2, the Shirogane clans' Master Puppet's chakra is said to be comparable to that of a tailed beast.
- Shukaku, Matatabi, Gyūki, and Kurama are based on popular animal yōkai from Japanese myth (tanuki, nekomata, ushi-oni, and kitsune respectively). The others are original creations by Masashi Kishimoto, mixing together various elements from certain species of animals.
- Shukaku is the only tailed beast that hasn't been shown giving its jinchūriki the characteristic red chakra-shrouds (Version 1 or Version 2 forms). This is presumably because Shukaku appeared early in the story when Kishimoto have not yet defined the specific abilities that all the tailed beasts share.
- While most of the tailed beasts have conventional tails, Chōmei and Gyūki differ; Chōmei has one regular tail and six wings that make up its seven tails, while Gyūki has eight cephalopod limbs that serve as tails.
- Shukaku and Matatabi are the only tailed beasts made from an inorganic material. Strangely, both were also believed to be a living ghost (生 霊, ikiryō).
- Despite having names, the tailed beasts do not generally use them, either for themselves or for each other, instead referring to each other by how many tails they have. Son Gokū is the only consistent exception, taking offence to the monikers.
- Shukaku also refers to itself only by its name, however it doesn't bother to be called the One-Tail.
- There have been cases that both the tailed beasts and their jinchūriki are affected to a certain degree during a full moon:
- According to Gaara, Shukaku's blood-lust is especially strong during a full moon.
- In the anime, Blue B, during his time as Gyūki's jinchūriki, noted that he would have trouble sleeping during the nights of the full moon, because anytime he would stare at it, it would reflect the loneliness in his heart. Gyūki would take advantage of those moments of weakness and try to gain control over Blue B's body. Blue B was also shown to take sleeping pills to help with this side-effect.
- Several non-jinchūriki shinobi have been said to hold chakra levels or strength comparable to that of a tailed beast. Such individuals include Madara Uchiha, Hashirama Senju, the Third Raikage, the Fourth Raikage, and most notably, also dubbed 'The Tailed Beast without a Tail', Kisame Hoshigaki. 
- The Madara Uchiha's Complete Body — Susanoo and Hashirama's Wood Release: Wood Human Technique are said to have destructive powers that rivalled the tailed beasts.
- In the anime, the six tailed beasts that were used by Tobi shared a Japanese voice actor with one of each other's respective jinchūriki.
- How the tailed beasts speak has been depicted inconsistently throughout the series, where sometimes their mouths move with their words and sometimes they don't.
- Due to the death of its jinchūriki, Isobu is the first known tailed beast to have "died" and revived.
- Years after the Fourth Shinobi World War, Konohagakure and Sunagakure make stuffed toys of Kurama and Shukaku, respectively. For Parent and Child Day, a limited-edition Kurama doll was offered, and toy stores in Konoha imported Shukaku dolls to sell for the holiday. In the anime, chibi-style masks of several of the tailed beasts were also on sale.
- Naruto chapter 404, page 14
- Naruto chapter 256, page 14
- Naruto chapter 467, pages 14-17
- Naruto chapter 510, pages 11-12
- Naruto chapter 670
- Naruto chapter 572, pages 12-13
- Naruto: Shippūden episode 464
- Naruto chapter 671, page 8
- Naruto chapter 648, pages 10-12
- Naruto chapter 568, page 13
- Naruto chapter 468, page 6
- Naruto chapter 592, pages 10-11
- Naruto chapter 609, pages 13-17
- Naruto chapter 654, pages 14-15
- Naruto chapter 660
- Naruto chapter 699
- Naruto chapter 692, pages 6-7
- Naruto chapter 318, page 12
- Naruto chapter 638, page 13
- Naruto chapter 572, page 10
- Naruto chapter 519
- Naruto chapter 503, pages 14, 17
- Naruto chapters 642-643
- Fourth Databook, page 93
- Naruto chapter 496, page 10
- Naruto chapter 568, page 10
- Naruto chapter 572, pages 6-7
- Boruto episode 55
- Naruto chapter 698, page 4
- Naruto chapter 91, page 19
- Naruto: Shippūden episode 318
- Naruto chapter 589, page 7
- Naruto chapter 468, page 9
- Fourth Databook, page 299
- Naruto chapter 641, page 5
- Naruto Shinden: Parent and Child Day
- Boruto episode 95