Conversation so far:
what are the dragons that the nazgul ride
The Fell beasts, also called hell-hawks and Nazgûl-birds, were the flying creatures on which the Nazgûl rode after being unhorsed at the Ford of Bruinen.
what did the fell beasts look like
The creature possessed a naked body without feathers, a long neck, and a vast hide between its horned fingers. Furthermore, the body of the creature gave off a stench.

Question: what is the origin of the beasts
Self-contained rewrite: what is the origin of the fell beasts