Tailed Beast

In the Naruto universe, there are nine giant demons called the that inhabit the planet. They are actually living chakra in physical form, often referred to as "Chakra Monsters". The Akatsuki organization has a plan that involves the capture of the Tailed Beasts.

History
The Tailed Beasts have been around 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 Wars, Hashirama Senju, the First Hokage, managed to capture and control them with his Wood Release techniques, and used them as a peace treaty 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. Their attempts to capture them and harness their immense power failed as no nation could control such power. Instead, the nations moved to capture the Tailed Beasts and seal them 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 host in their attempt to create a weapon powerful enough to wipe out an entire country. The only two Tailed Beasts that are not currently sealed are the Eight-Tails and the Nine-Tailed Demon Fox.

Legend of the Tailed Beasts
According to some fans, the "Legend of the Tailed Beasts" is an epic 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 on Chinese websites, some suspect this chinese site.

The veracity of the legend breaks when it is cross-referenced with non-fandom sources on Japanese folklore. For example, the isonade, which is held to be the Three-Tails in this account, is in fact equipped with only one large, hook-covered tail, and the "houkou" which is actually a Chinese creature called penghou, named as the Five-Tails, has no tail at all. The badger was said to be the lord of all these beasts in the original legend, and Naruto fans have claimed certain Bijū to be a badger instead of it's true form which was a tanuki for Ichibi no Shukaku and a bat for Shichibi no Makumore)

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.