Not 'night horse' — the 'mare' is from Old English 'mære', a demon that sits on sleepers' chests, causing suffocation and terror. From Proto-Germanic *marǭ (incubus), possibly PIE *mer- (to harm/die). The same creature appears in French 'cauchemar', German 'Nachtmahr', and Norse 'mara'.
A frightening or unpleasant dream; a terrifying or very unpleasant experience or prospect.
From Middle English 'nihtmare', combining 'niht' (night) and 'mare' — but 'mare' here is NOT the word for a female horse. It comes from Old English 'mære', a supernatural being or incubus that was believed to sit on a sleeper's chest, suffocating them and causing terrifying visions. From Proto-Germanic *marǭ (incubus, nightmare spirit), possibly from PIE
The 'mare' in nightmare has nothing to do with horses. It's a demon. Henry Fuseli's famous 1781 painting 'The Nightmare' shows both meanings: a demonic incubus crouching on a sleeping woman's chest, while a horse (mare) peers through the curtains — a visual pun on the word's false etymology. The painting created the modern image of nightmares; the word