Flying
horses?
You must have been watching the
Rankin-Bass ROTK again, haven't you?
(If you don't know, the "
fell beasts" were pictured as horses with wings in that cartoon, except for
the one ridden by the
witch-king).
They were more of a pterodactyl-type creature, and I don't think they were corrupted eagles--they were probably something else that
Sauron nursed with "fell meats." Some prehistoric monster, like
the Watcher in the Water. No mention is made of them in the
tales about Beliarand, so I don't think they have anything to do with Morgoth--unless they came out of the pits of
Utumno (which is actually likely).
They weren't magical, for one thing: while darts are described raining down from these beasts in ROTK, these are fired by the
Nazgul and have nothing to do with the beasts themselves.
And as for "chummish" behaviour between Sauron and
Saruman, you'll notice neither Sauron nor Saruman were on very good terms in the book. Saruman wanted to set up himself as the new lord of
Middle-Earth, in opposition to Sauron: but in the end fell under his control until he was nothing but a mere slave. Saruman never bred
Uruk-hai...those apeared before Saruman turned
evil, when
the east bank of
Osgiliath fell. Uruk-hai were really just the warrior-caste of
orcs, well-trained and far more scary. And no, no cross-breeding with humans was involved. However, Saruman did breed man-orcs (seperate from Uruk-hai) on his own, and these were the guys who apeared in the
scouring of the Shire.
So, in short, Saruman couldn't have helped breed the "fell beasts", because they were grown in
Mordor over a long period of time, before Saruman even thought of perhaps taking
the ring for himself.
What were they, then? Corrupted
Eagles is one choice. Prehistoric hell-creatures is another (the one I favour)--otherwise, pterodactyls.