[Ibrîniðilpathânezel]

Hmmm....

Going out on a limb here (while waiting for the Christmas fudge to cool): The thought comes to mind that whatever the creature's real name might be, the original critter that eventually became the Winged Nazgul Mount™ could have been Melkor's attempt to parody his brother Manwe's Eagles. I don't recall ever reading that in any of Tolkien's various writings and drafts, but it would go along with him making parodies of Elves and Ents (it also keeps with the nice alliterative pattern). That it looks nothing at all like an eagle, save for having wings and claws, would not be amiss, since orcs and trolls about as similar to Elves and Ents.

And I still tend to think that we didn't see them used sooner because either the few that were around weren't yet capable of being effective mounts, or because Sauron was holding them back to increase the fear factor when he needed it.