Chakra beasts

Chakra beasts (チャクラじゅうるい, Chakura Jūrui) are incredibly large creatures, living in the Land of Origins, with massive sources of chakra. The Ten-Tailed Beast was one such creature. These creatures are exceedingly rare and each possess different qualities. Like their cousins, the, each of these creatures possesses an almost otherworldly level of chakra and are capable of massive amounts of destruction. Unlike the tailed beasts, who require hosts to have intelligence, the chakra beasts seem to have human-level intellect even without hosts. Some seem to have traits similar to kekkei genkai, such as the Ten-Tails' unknown dōjutsu, which they can pass on to their host and, from there, be passed on to future generations; a trait they do not share with their tailed beast cousins. Unlike the tailed beasts, who are ranked in power in accordance to the number of tails they possess, each chakra beast has exactly the same amount of chakra, regardless of the number of tails they possess. Like the tailed beasts, on the other hand, they can be sealed into human vessels which are also called jinchūriki (人柱力, Power of Human Sacrifice), as the Sage was called the first jinchūriki, despite not having a tailed beast sealed within him.

History
During the days of the, these creatures often attacked human settlements, causing tremendous damage. Once the Sage learned to use chakra like the beasts, he sealed one of them, the Ten-Tails, within himself to prevent any further disasters. The Sage was then able to force the other beasts away for some time. Upon his deathbed, he feared that the return of the Ten-Tails would be a beacon for the beasts to return to known civilization so he spit its chakra up into and scattered them across the world, sealing the body of the creature with his. Thousands of years later,, a descendant of the Sage, fuses the beasts back into the Ten-Tails, reviving it and causing the chakra beasts to return to the known world.

After their return, it became necessary to seal them in order to prevent them from attacking civilization again. So, when one appeared, it was sealed into a newborn child as the tailed beasts once were.

Ten-Tailed Beast
The Ten-Tailed Beast (十尾, Jūbi) is the original, primordial demon of the Naruto universe. All nine of the tailed beasts are but portions of chakra divided from the Ten-Tails. Madara Uchiha's ultimate goal, the, was to capture and merge all nine of the tailed beasts back into the Ten-Tails, and become its jinchūriki to cast the reflection of his Sharingan on the moon, casting a genjutsu known as the Infinite Tsukuyomi which would put the entire world under his control.

Trivia

 * The Ten-Tails is the only beast referred to as a "demon".