D&D 5E - What's the origin of the Nightmare?
See: http://en.wikipedia.org/wiki/The_Nightmare think it is the best you are going to get.Etymology
The word "mare" comes (through Middle English mare) from Old English mære, mare, or mere, all feminine nouns. These in turn come from Common Germanic *marōn. *Marōn is the source of Old Norse: mara, from which are derived Swedish: mara; Icelandic: mara; Faroese: marra; Danish: mare; Norwegian: mare/mara, Dutch: (nacht)merrie, and German: (Nacht)mahr. The -mar in French cauchemar ("nightmare") is borrowed from the Germanic through Old French mare.
The word may ultimately be traced back to the reconstructed Proto-Indo-European root *mer-, "to rub away" or "to harm". Hungarian folklorist Éva Pócs endorses an alternate etymology, tracing the core term back to the Greek μόρος (Indo-European *moros), meaning "death".
In Norwegian and Danish, the words for "nightmare" are mareritt and mareridt respectively, which can be directly translated as "mare-ride". The Icelandic word martröð has the same meaning (-tröð from the verb troða, "trample", "stamp on", related to "tread"), whereas the Swedish mardröm translates as "mare-dream".
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