What is the etymology of the word "nightmare"?

Evil spirits affecting men and horses in their sleep.

"Nightmare" is Anglo-Saxon in origin and dates to around the year 1300 C.E. The word is a compound of "night" (dark part of the day) and "mare" (night-goblin, incubus, succubus) in the sense of an evil spirit afflicting either men or horses as they slept, often with feelings of suffocation.

A few hundred years later and the succubus was lost, in favour of just those suffocating sensations.

By 1829 C.E., nightmare came to mean "any bad dream" or "distressing experience."

https://www.etymonline.com/word/nightmare