The ten Bijuus (尾獣, bijū) are the ten titanic behaemoths 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 "Chakra Monsters" (チャクラのバケモノ, Chakura no Bakemono), 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 Bijuus
Shukaku (守鶴, Shukaku), more commonly known as the One-Tail (一尾, Ichibi), is a tailed beast that was last sealed within Gaara of Sunagakure after being sealed in two other hosts. It has been captured by Deidara, then extracted and sealed by Akatsuki.
Matatabi (又旅, Matatabi), more commonly known as the Two-Tails (ニ尾, Nibi), is a tailed beast that was sealed within Yugito Nii of Kumogakure. It has been captured by Hidan and Kakuzu, then extracted and sealed by Akatsuki. It was later resealed into a reincarnated Yugito during the Fourth Shinobi World War.
Kokuō (穆王, Kokuō), more commonly known as the Five-Tails (五尾, Gobi), is a tailed beast sealed within Han from Iwagakure. It has been captured, extracted, and sealed by Akatsuki.
In the anime, Kokuō is indicated to have been one of the two tailed
beasts that Akatsuki captured before the start of Part II. It was later
resealed into a reincarnated Han during the Fourth Shinobi World War.
Chōmei (重明, Chōmei), more commonly known as the Seven-Tails (七尾, Nanabi), is a tailed beast sealed within Fū from Takigakure. It has been captured, extracted, and sealed by Akatsuki.
In the anime, Chōmei is indicated to have been one of the two tailed
beasts that Akatsuki captured before the start of Part II. It was later
resealed into a reincarnated Fū during the Fourth Shinobi World War.
Gyūki (牛鬼, Gyūki), more commonly known as the Eight-Tails (八尾, Hachibi), is a tailed beast sealed within Killer B of Kumogakure after Fukai, Fukai's uncle and Fukai's father failed before him.
Kurama (九喇嘛, Kurama), more commonly known as the Nine-Tails (九尾, Kyūbi; English TV "Nine-Tailed Fox"), is a tailed beast that was first sealed into Mito Uzumaki after the battle at the Valley of the End, then into Kushina Uzumaki after Mito's death. During its attack on Konoha, Minato Namikaze sacrificed his life by splitting the fox's chakra into two: sealing the Yin half within himself and sealing the Yang half within his son.
The Ten-Tails (十尾, Jūbi) is the primordial entity and original tailed beast tied to the legend of the Sage of the Six Paths. Its chakra was divided into the nine tailed beasts and its body sealed into what would become known as the moon,
by the Sage in his final moments of life. The Ten-Tails is considered
to be the creator of the world, and the herald to its end should it ever
be revived. Obito Uchiha's ultimate ambition, which was created and designed by Madara Uchiha, the Eye of the Moon Plan, is to capture and merge all nine of the tailed beasts' chakra back into the Ten-Tails' body, and become its jinchūriki and then cast the reflection of his Sharingan on the moon, casting a genjutsu known as the Infinite Tsukuyomi which will put the entire world under his control.
