In his prelimary sketches of LoTR (HoME 8) Tolkien describes the 'flying machines' [img]smilies/wink.gif[/img] of the Nazgul was being eagles/eagle like being, some kind of vulture.
Quote:
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The Black Riders now have taken form of demonic Eagles and fly before host, or (take Eagle like) vulture birds at steeds
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Though this is just a prelimanry sketch, in which apparently the Nazgul could shapeshift, into a eagle like being/s, they still rode steeds that were like Vultures, NOT dragons or drakes.