[Ibrîniðilpathânezel]

I wonder, though: do we have any information as to where the so-called fell beasts came from? Could they possibly have been something Sauron was working on relatively recently, breeding in Mordor? Perhaps they were his attempt to bring back the strength of the dragons of old, but being of lesser craft and power than Morgoth, this was the best he could do, and they simply hadn't been sufficiently mature to carry a rider until that point in the Third Age. That's one thought.

Another might be that the Nazgul at first were out trying to ascertain the location of the Ring, and if they'd been swooping about on these hideous winged mounts, they might not have received even as pleasant a reception as they got from Farmer Maggot and the Dwarves of Erebor. A creepy black rider on a black horse might give one pause, but the same creepy black rider flying in on a critter disturbingly reminiscent of a dragon (especially to the Dwarves) would inspire doors to be locked and barred even before the thing landed, methinks.