Dwimmerlaik
Dwimmerlaik was a name given to Lord of the Nazgûl by Éowyn (as Dernhelm) when she confronted him during the Battle of the Pelennor Fields[1].
By extension, it is possible that Dwimmerlaik is how the Rohirrim called the rumors of the Nazgûl in general.
Etymology
It means 'phantom' (lit. "magical art") cf. Dwimordene, Dwimorberg; from the Old English word gedwimer 'sorcery'.
It is not clear whether the word refers to a creature of the Rohanese folklore (like the holbytlan) therefore when Eowyn confronted the Witch-King she rather identified him with one, than coining the word on that moment.
David Day in A Tolkien Bestiary considered that a Dwimmerlaik is any creature of Rohanese superstitious folklore that includes Elves and Ents. However there is no indication in Tolkien's writings that the word pre-existed or that it was used for other races[2].
See also