Tailed Beast

The are nine giant demons that serve to drive the plot of the Naruto series. They are differentiated by the number of tails they have, with each possessing an amount from one to nine. The tailed beasts are large, living forms of chakra, sometimes referred to as, giving them power that far outmatches most shinobi. However, this immense strength is tempered due to their bestial nature, which restricts them from being intelligent enough to use it effectively. When the tailed beasts are sealed within humans, they are able to benefit from the knowledge of their hosts, giving them the ability to speak and better utilize their powers. Akatsuki has been capturing the tailed beasts and their hosts, and as of the Fourth Great Shinobi World War possess seven of the nine beasts.

History
The tailed beasts have been around since long before the founding of the ninja villages. In the beginning of the ninja history, there existed one demon, the Ten-Tailed Beast, that plagued the planet until the Sage of the Six Paths defeated and sealed the beast within himself, making him the first jinchūriki. On his deathbed, he divided the Ten-Tails' chakra into the nine other tailed beasts and sealed its body in the moon.

By the time of the formation of the shinobi villages, Hashirama Senju, the First Hokage, managed to capture and control some of 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.

The Tailed Beasts
{{#arraymap:One-Tailed Shukaku, Two-Tailed Monster Cat, Three-Tailed Giant Turtle, Four-Tailed Monkey, Five-Tailed Dolphin Horse, Six-Tailed Slug, Seven-Tailed Horned Beetle, Eight-Tailed Giant Ox, Nine-Tailed Demon Fox, Ten-Tailed Beast|,|!|

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Trivia

 * Only four people are said to have full control of the tailed beasts. They are Madara Uchiha (through his Eternal Mangekyō Sharingan), Hashirama Senju (through his special Wood Release technique), and two jinchūriki: Yagura and Killer Bee.
 * The Seven-Tails and the Eight-Tails do not actually have tails. The Seven-Tails has six insect wings and one tail connected to the tip of its abdomen, while the Eight-Tails has eight cephalopod arms.
 * In the second Shippūden movie, it features a creature called the Zero-Tailed Masked Leech. Despite having a numerical tail name, it is never called a tailed beast and has characteristics that are quite different from the real tailed beasts.
 * Several non-jinchūriki shinobi, such as Kisame Hoshigaki and the Fourth Raikage, Ē, have been said to hold chakra levels comparable to that of a tailed beast.
 * In the anime, after the sealing of the Two-Tailed Monster Cat, Pain stated there are four tailed beasts remaining. Later in the anime after the Three-Tailed Giant Turtle and Four-Tailed Monkey were sealed (making it two tailed beasts remaining) and Taka joins forces with Akatsuki, Kisame states there are three tailed beasts remaining even though half of the remaining four were sealed.
 * According to some fans, the Legend of the Tailed Beasts is a story from Japanese folklore which Masashi Kishimoto, the author of Naruto, draws from to create the tailed beasts in his work. However, it is an example of internet-based fake lore which originated from this Japanese site. The veracity of the legend breaks when it was cross-referenced with non-fandom sources on Japanese folklore. For example, the isonade, which was thought to be the Three-Tails, is in fact equipped with only one large, hook-covered tail; and the hōkō, which was thought to be the Five-Tails, actually has no tail at all. Additionally, the author of the story admitted it was not a true legend, unfortunately it has gone largely unnoticed by most fans of the story.