what is it that the nazgul ride ?

[gayare'dion]

is it some type of giant bird ? or a small dragon, or winged lizard-like creature ? does it even have a name ? i tried searching the encyclopidia of arda, but didnt really find anything.

[Gimli Son Of Gloin]

It doens't give a name. They are juat winged beasts. And rather large at that.

[Minyacirith]

Nobody knows B/C nobody could approach them.

Or if somebody knows, they aren't important in the LotR books.

[Aldagrim Proudfoot]

ROTK Battle of the Pelennor Fields

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 forgottren mountains beneath the Moon, outstayed their day, and in its hideous eyrie bred this last untimely brood , apt to evil.

It was some bird Sauron bred for the Nazgul.

[AragornsHeir]

I would go with the dragon creatures. They most likely bread them to bear the Nazgul just like those horses were bread specialy for them. When the horses failed them at the ford they must have replaced them with the dragons.

[The Fifth]

Like Aldagrim here put, I go with the description in the books. I imagine them as big, ugly, featherless birds.

[Angmar_the_Horrible]

I mostly agree with The Fifth, large, featherless birds...Or some kind of flying
dinosaur...
I am sure you know the painting by John Howe,
"Nazgul returning to Barad-Dur": his
painting of the flying steed comes very close
to my imagination.