Talk:Nightmare/@comment-34693780-20180507161120/@comment-29602226-20180507182900

Actually, the origin of the word "nightmare" is probably celtic goddess Epona, a warrior goddess of horses, death in battle, dreams and an afterlife guide. She would appear in dreams to warriors the night before the battle that would claim their lives, usually as a mare...

So "nightmares" are, quite literally, "night mares", as in, "dream-visions of mare goddess Epona"