Board Thread:Theories and Speculation/@comment-26422475-20181104100351/@comment-33485854-20181114191657

They are Chakra given Form. Hagoromo created them through the use of Yin-Yang Release, dividing Kaguya's Chakra into 9 separate pieces and then granting them sentience. They are Chakra like Fire Release is Chakra. Imagine a Rasengan with the ability to sustain itself.

Their Chakra itself likely holds its sentience, explaining the "Will" in Tailed Beasts' Chakra and how they are able to reform after dispersal.