The first problem with the theory of
Morgoth corrupting
Eagles is that it never appears in his writings that
Tolkien intended for
The Silmarillion, whereas
Orcs, whether from
Men or
Elves, does. Also the
Olog-hai &
Trolls in general are both stated in TLOR as having the source of their origins.
Then, the ancient Eagles had
spirits sent into them from
Manwe - how is that noble spirit going to be corrupted against it's will?
Ruined perhaps outwardly like Elves captured at
Angband, but not changed in the 600 or less years into Orcs either. Look at Gwindor in the
Turin tale. A mere shell of himself, but an
Elf none-the-less. And, 600 years is all Morgoth would've had to ruin a higher 'spiritual' being in these Eagles. Not possible.
Finally, as in most cases, the
books CAN actually give us the answer:
ROTK, Battle of the Pelannor
Fields:
'it was a winged creature; If bird, then greater than all
birds, and it was naked, and neither quill nor feather did it bear, and it's vast pinions were as webs of hide between horned fingers; and it stank (Sounds like a few former co-workers of mine).'
Note this part:
'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.'
The Silmarillion, 'Of The Beginning Of Days':
'nonetheless the evil of
Melkor and the blight of his hatred flowed out thence (from
Utumno) and the Spring of
Arda was marred. Green things fell sick and rotted, and
rivers choked with weeds and slime, and fens were made, rank and poisonous, the breeding place of flies; and forests grew dark and perilous, the haunts of fear; and beasts became
monsters of horn and ivory and dyed the earth with blood.' - sounds an awful lot like Tolkien describing what scientists today call 'prehistoric times', no?
Then:
The Two
Towers, '
The White Rider':
Gandalf: "Far, far below the deepest delvings of the
Dwarves, the world is gnawed by nameless things. Even
Sauron knows them not. They are older than he. Now I have walked there, but I will bring no report to darken the light of day." - Note: This IS a mountain Gandalf is referring to.
Finally:
The Letters Of JRR Tolkien #211:
Q: Did the
Witch-king ride a pterodactyl at the
siege of Gondor?
A: Yes and no. I did not intend the steed of the Witch-King to be what is now called a 'pterodactyl', and is often drawn (with rather less shadowy evidence than lies behind many monsters of the new and fascinating semi-scientific mythology of 'Prehistoric - me: lol). But obviously it is
pterodactylic and owes much to the new mythology, and it's descrription even provides a sort of way in which it could be the last survivor of older geological
eras.
I had no idea what letter #211 would say, just recalled that there was something in the Letters about a 'pterodactyl'......
So, the winged steed is an 'ancient evil' (nameless thing) from the time of Melkor's evil preverting
Middle-earth. Not a one on one perversion like with
Glaurung, but a massive outpouring of the much mightier, just came to Arda, Melkor who could 'rear up the
Towers of Mist to hinder
Orome's ridings' and who's mere pressence in Utumno spread forth & perverted, water,
plants and
animals alike.
These winged steeds were obviously part of the perversion - from eagles, perhaps, indeed, probably. But not 'taken by Morgoth and perverted' like he did Men and/or Elves into Orcs or Glaurung into the first
dragon.
It just happened as a residue of Melkor's evil dripping out of the area around Utumno; no 'hands on project'.
Edit: And, might I add, long before Manwe sent 'spirits' to live in the eagles that lived in Crissaegrim and/or
Thangorodrim.