My take on the Wainriders is that Tolkien drew upon various historical cultures
in developing these people. They are, moreover, a "confederacy" of
various peoples from the East, so Tolkien was able to create a somewhat
culturally non-specific enenmy for Gondor that could evoke elements of
Egyptian, Scythian, Hittite, and other waggon and chariot-riding warriors.
Side note, since when Eärnil defeated them many were lost in the Dead Marshes,
I wonder if any of their faces would have been among those that Frodo and Sam
would see, or if there is a wain lying submerged in there for achaeologists to
dig
out!
Most people have made this mistake of thinking Middle-earth is a particular
kind of Earth or is another planet of the science fiction sort but it's just
an old fashioned word for this world we live in, . . . at a different stage of imagination. --Tolkien's Last Radio Interview
