Despite their size, they were not supernatural beings, but living creatures, and they could be hurt and slain: Legolas shot one with an arrow on the River Anduin, and Éowyn slew another on the Pelennor Fields.
Whatever the nature of the Winged Creatures, it seems that those used by the Nazgûl were the last of their kind in Middle-earth, and had been grown to an unnatural size by Sauron. Perhaps he was describing their wheeling, watchful flight, rather than the appearance of the Creatures themselves. In Ithilien, though, Frodo compared these beasts to giant carrion birds, a comparison that doesn't appear to match the other more detailed description. That ridden by the Lord of the Nazgûl at the Battle of the Pelennor was described as naked and featherless, with wings of hide like a bat, and a long, sinuous neck.
Fetid black creatures raised by Sauron as flying mounts for his Ringwraiths.
