The Tailed Beasts have been around way before the First Great Shinobi War. Because of their immense power, the shinobi from each nation tried to use the Tailed Beasts as military weapons during the Shinobi Wars. After the first war, Hashirama Senju, the First Hokage, managed to capture and control them with his Wood Release techniques, and used them as peace treaties to the other great shinobi nations, in order to stabilize the balance of power between them. After his death, however, the shinobi nations couldn't control these giant demons themselves and they began to run wild, so the nations instead moved to seal the Tailed Beasts within humans, which they were called Jinchūriki. This method worked by the time period of the series.
Akatsuki is currently in the process of extracting the Tailed Beasts from their hosts in their attempt to create a weapon powerful enough to instantly wipe out an entire country. The only Tailed Beasts that are not currently sealed are the Eight-Tailed Giant Ox and the Nine-Tailed Demon Fox.
Hosts and their Demons:-
|Host Name||Hails from||Demon Name||Species||Status||Sealed in host by||Unique Traits of the Demon||Forms|
|Naruto||Konohagakure||Nine-Tailed Demon Fox||Kitsune||Active||Minato Namikaze(Yondaime Hokage)||Near-limitless chakra, Massive strength||Naruto Jinchūriki Forms|
|Kirābī||Kumogakure||Eight-Tailed Giant Ox||Ushi-Oni||Active||Unknown||Missing Left Horn||Kirābī Jinchūriki Forms|
|Unknown||Takigakure||Seven-Tailed Beast||Unknown||Captured and extracted by the Akatsuki, deceased||Unknown||Unknown|
|Unknown||Unknown||Six-Tailed Beast||Unknown||Captured and extracted by the Akatsuki, deceased||Unknown||Unknown|
|Unknown||Iwagakure||Five-Tailed Beast||Unknown||Captured and extracted by the Akatsuki, deceased||Unknown||Unknown|
|Rōshi||Iwagakure||Four-Tailed Beast||Unknown||Captured and extracted by the Akatsuki, deceased||Unknown||Unknown|
|Unnamed Shinobi*||Kirigakure||Three-Tailed Giant Turtle||Turtle||Captured and extracted by the Akatsuki||Unknown||Single, scarred, opened left eye,High-speed swimming|
|Yugito Nii||Kumogakure||Two-Tailed Monster Cat||Bakeneko||Captured and extracted by the Akatsuki, deceased||Unknown||Pyrokinesis,Blue flame body||Yugito Jinchūriki Forms|
|Gaara||Sunagakure||One-Tailed Shukaku||Tanuki||Captured and extracted by the Akatsuki||Chiyo||Fires concentrated blasts of wind mixed with chakra,Near-impenetrable sand defense||Gaara Jinchūriki Forms|
Note:*In the past, the Three-Tails was imbued with an unnamed shinobi from Kirigakure, later it seemed to have been extracted from its Jinchūriki for reasons unclear, and was set free in the wild. After which, the Three-Tails lived in a lake and remained uncaptured by any shinobi. According to Tsunade, the Three-Tails was lost after the great war.
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Jinchūriki are humans that have a Tailed Beast sealed within them. They exhibit extraordinary powers, and in some cases are wiser than their Tailed Beasts because they have the intelligence to use the beast's power effectively.