Ten-Tails

According to Madara Uchiha, the is the ultimate fusion of the other nine tailed beasts. So-far, the Ten-Tailed 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 different orbits around the pupil.

It was originally one being, but the Sage of the Six Paths sealed it into his own body, becoming the first Jinchūriki, and saving the world. If the Sage were to die, the beast's foul and powerful chakra would break the seal, thus releasing the beast upon the world once again. To prevent this, upon his deathbed, he divided the Ten-Tailed beast's chakra into the nine monsters of pure chakra, known as the tailed beasts, throughout the world, and sealed its actual body through the usage of the Chibaku Tensei, creating the moon. Madara's current goal, the Eye of the Moon Plan, is to capture all nine tailed beasts and merge them back into the ten-tailed beast and become its host to control all the people in the world.