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.
what is the origin of the fell beasts
While the exact origin of the beasts is unknown, they were likely bred by Sauron from a creature of the Elder Days to supply the Nazgûl with steeds.
how were the fell beasts portrayed in lord of the rings
One of the Nazgûl is shown riding a fell beast. However, Bakshi's film only covers events up to the Battle of the Hornburg.

Question: how about in return of the king
Self-contained rewrite: how were the fell beasts portrayed in return of the king