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 (尾獣, bijū) 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 "Chakra Monsters" (チャクラのバケモノ, Chakura no Bakemono),[1] 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
