Ten-Tails

According to Madara Uchiha, the is the original demon; all nine of the tailed beasts ultimately are just aspects divided from the whole; they all originally existed as just one being. So-far, the Ten-Tailed Demon Beast has only been seen once, in silhouette, but it's body seems to be closest to the form of the Nine-Tailed Demon Fox's body. It also has only one eye; this single eye is full of tomoe in three different orbits around the pupil.

In his lifetime, the Sage of the Six Paths saved the world from the Ten-Tailed Demon Beast, defeated and captured the Ten-Tails and imprisoned it by sealing it into his own body, becoming the first Jinchūriki in history. However, the Sage knew that if and when he were to die, that the Ten-Tailed Beast would not die with him; without the Sage's own chakra to keep the Ten-Tails suppressed, the Beast's foul and powerful chakra would break through the seal, thus releasing the Beast to walk upon the unsuspecting world once again.

The Sage sought to prevent this and took measures to do so; as the Sage reached the end of his life, he used his remaining time and divided up the chakra of the Ten-Tailed beast's into nine seperate, monsterous constructs of pure chakra, and then banished them all to the corners of the world. These nine constructs of living chakra would become known throughout the world as the Tailed Beasts. As a further measure against it's return, the Six-Paths Sage used the Chibaku Tensei technique and sealed the actual physical body of the Ten-Tailed Demon Beast within a prison of compressed stone and positioned it far up in the Heavens, visible to everyone but far above anyone elses reach--resulting in the moon.

Madara Uchiha's ultimate goal, his Eye of the Moon Plan, is to capture and reunite all nine of the tailed beasts and merge them back into the ten-tailed beast, and become its host to control all the people in the world.