Jubi is actually the fusion of the the 9 tailed beasts that possess the strongest chakra. The chakra was great the it would break once the Rikudo Sage died and would once again wreak in havoc to the land.

When the Rikudo Sage was dying, he used his remaining chakra to divide the Jubi’s chakra and disperse them through out the land. That’s how the 9 tailed beasts started to exist.

The 9 tailed beasts are actually just the division of Jubi’s chakra.

And after that, the Jubi’s empty body was sealed away and sent it to the sky where no one could reach it and it became the moon.