Were the Wraiths originally of Númenorean stock? If so, it might have taken a long time before the rings utterly corrupted them.
I think that's a given, Teresa - Gandalf actually says:
FotR, Book 1, Ch. 2, The Shadow of the Past: my emphasis.Nine he gave to Mortal Men, proud and great, and so ensnared them.
I doubt that, at that time and in that place there were any other men in Middle-earth that the phrase would apply to...