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 beast, the Ten-Tailed Beast, that plagued the planet until the Sage of the Six Paths defeated and sealed the beast inside himself. 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, which were called jinchūriki.

One-Tailed Shukaku
It has been captured by Deidara, then extracted and sealed by Akatsuki.

Two-Tailed Monster Cat
It has been captured by Hidan and Kakuzu, then extracted and sealed by Akatsuki.

Three-Tailed Giant Turtle
It has been captured by Deidara and Tobi, then sealed by Akatsuki.

Four-Tailed Monkey
It has been captured by Kisame Hoshigaki, then extracted and sealed by Akatsuki.

Trivia

 * Only four people are said to have full control of the tailed beasts. They are Madara Uchiha (through his Eternal Mangekyo Sharingan), Hashirama Senju (through his special Wood Release technique), and the 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.
 * According to some fans, the Legend of the Tailed Beasts is a story from Japanese folklore which Naruto author Masashi Kishimoto draws from to create the tailed beasts in his work. However, it is an example of internet-based fakelore 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.