Other Creatures to Ride -- Reading Material

What else can you ride, besides horses and wolves/Wargs?  The Nazgûl found something.

"The Winged Messenger!" cried Legolas.  "I shot at him with the bow of Galadriel above Sarn Gebir, and I felled him from the sky.  He filled us all with fear.  What new terror is this?"

"One that you cannot slay with arrows," said Gandalf.  "You only slew his steed.  It was a good deed; but the Rider was soon horsed again.  Her he was a Nazgûl, who ride now upon winged steeds.

The Two Towers, "The White Rider"

Sam:  "The Riders are about again, up in the air.  I saw them.  Do you think they could see us?  They were very high up.  And if they are Black Riders, same as before, then they can't see much by daylight, can they?"

"No, perhaps not," said Frodo.  "But their steeds could see.  And these winged creatures that they ride on now, the can probably see more than any other creature.  They are like great carrion birds.  They are looking for something."

The Two Towers, "The Black Gate is Closed"

The great shadow descended like a falling cloud.  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 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.  Down, down it came, and then, folding its fingered webs, it gave a croaking cry, and settled upon the body of Snowmane, digging in its claws, stooping its long naked neck.

Upon it sat a shape, black-mantled ... the Lord of the Nazgûl,

Return of the King, "The Battle of the Pelennor Fields"