LOTR: Where Did The Fell Beasts, The Flying Servants Of The Nazgul, Come From?

gamerant.com:

Fell beasts, the flying creatures that carry the Nazgul after their black horses are lost at the Ford of Bruinnen, are some of the most fearsome creatures in all of Middle Earth. Little is known about them, other than the fact that their piercing shriek (which is actually made up of Fran Walsh screaming, over-layed with several other soundscapes in the films) has been known to stop dead the beating hearts of men. So where did these cruel beasts come from?

There are a few names by which the creatures are known, including Hell-hawks, and Wraith-birds, but the term ‘Fell Beast’ itself doesn’t refer just to these creatures alone. It is also used to describe the evil things that crawl out of Mirkwood, like the spidery descendants of Shelob. ‘Fell’ simply means ‘terrible’ or ‘full of dread’ in the old English langauge, so the name can really be applied to any of Sauron’s creatures. Interestingly, many fans of the movies think that the creatures are, themselves, called Nazgul’s, but this is not the case.

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The Nazgul are the 9 ringwraiths of Sauron, and can sometimes refer to the Fell Beasts with their riders collectively, but never just to the creatures themselves. It is safe to assume then, that these beasts have been specifically bred and twisted to the dark lords will (like so many other creatures in the trilogy, including the trolls in each battle) so that they will always serve his evil purpose, and obey the 9 Ringwraiths who ride upon their backs. ‘And the Dark Lord took it, and nursed it with fell meats, until it grew beyond the measure of all other flying things, and he gave it to his servant to be his steed.’

In the Return of the King, the Fell Beasts are described as ‘a winged creature’ ‘naked,