Hi! (and Fell Beasts...)

Hey everyone! I'm new here, and this is such a great site!!

Okay my first question to everyone here has been on my mind for a week or so:

The Fell Beasts.
What are they? Some sources talk of prehistoric creatures forgotten by many and changed by the Dark Lord.

The great shadow descended like a falling cloud. and behold! it was a winged creature: if bird, then greater than all other birds, and it was naked, and neither quill nor feather did it bear, and its vast pinions were as webs of hide between horned fingers; and it stank. A creature of an older world maybe it was, whose kind, lingering in forgotten mountains cold beneath the Moon, outstayed their day, and in hideous eyrie bred this last untimely brood, apt to evil. And the Dark Lord took it, and nursed it with fell meats, until it grew beyond the measure of all other things that fly; and he gave it to his servant to be his steed. Down, down it came, and then, folding its fingered webs, it gave a croaking cry, and settled upon the body of Snowmane, digging in its claws, stooping its long naked neck. ... ( Love that passage..)


The highlighted bit above..

Okay this is what I've been thinking. You know how the orcs were made from Elves? Well what if the Fell Beasts were made from the great Eagles? It talks of it being 'like a Giant bird' with wings.
So does anyone know the actual origin of them? Anyone thinking along the same lines as me or am I far off the mark?

Thanks,
Dwimmerlaik.