“Of course!” I said to myself. “That’s why we have nightmares, not nightstallions.” But I was wrong again. The mare in the nightmare is not a member of the genus Equus which comes in female and male genders. No. The mare in nightmare comes from Old English mera or mære meaning goblin, the female kind. Not a horse in sight.
We have to go back to the resolutely masculine Latin culture before we discover the incubus, the masculine dream-entity that lies down and suffocates the sleeper. My favorite etymology dictionary dates it to 1200 as the evil protagonist of nightmares. We have to wait until the 1300s before he is joined by succubus, the female ghost that has sex with men in sleep—that word formed from the Late Latin succuba, a strumpet. Those familiar with Google’s Ngram Viewer† can easily discover that, whether ghost or flesh, males are always much more eager for sex than women:
--------------------†Ngram viewer is a Google facility that scans digitized books accessible to Google from 1800 to 2000 and counts the frequency of words used in these sources. You enter two words or phrases separated by a comma. The address of the facility is here.