Old English maere denotes an incubus-like night spirit, cognate with German Mahr and recorded from the medieval period in compounds like nightmare for the oppressed-sleep experience. The female horse mare descends from a different Old English word, mere, and the two merged in sound only. Sleep researchers now map the night-mare experience onto sleep paralysis with hypnagogic hallucination.
Folklore across Europe gave the mare doors and keyholes to slip through, knots in horses' manes where she rode them at night, called hag-riding in English, and remedies from knives under the pillow to salt. Henry Fuseli's 1781 painting The Nightmare, demon on the chest, horse in the curtain, sealed the horse confusion in paint.
The clinical reading says the demon was always sleep paralysis and culture supplied the costume. The folklore reading says the experience was universal and science merely renamed her. The word sits exactly on the seam between the two.
The word survived the demon. Now we use it for bad meetings. Is that progress, or amnesia?