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I dreamed that I was lying in a cool, open field of dewy grass, staring up at the stars in the night sky. Light began seeping in around the edges of the horizon — not just in the east, like a sunrise, but all the way around. The light at the edges grew inward, creating a hazy, undulating edge with star-filled night on the inside and blue sky on the outside. Brighter and brighter the sky grew as the night shrank toward the zenith, until only a small patch of dark sky remained.
The wavering edge separating day from night hardened, and I could see that the last remnants of night had taken the shape of a magnificent dark horse, and it was galloping toward me, the steam from its breath casting off nebulae and galaxies into infinite space.
And then I woke up.
The dream wasn't a nightmare — that has nothing to do with horses — it was just a horrible dream. Thin plot, no character development, predictable symbolism. Just horrible.
Back when the BCs were fixin' to turn into the ADs, people believed that horrible, frightening dreams were caused by evil demons who visited (and, ahem, consorted) while their victims slept. A woman was visited by a male demon called an incubus, and a man was visited by a female succubus. Both words come from Latin: incubus comes from in + cubare, "to lie upon," and succubus comes from suc (or sub) + cubare, "to lie under." (Apparently, demons stick to the missionary position.)
This belief extended into the Dark Ages. The Old English word for an incubus was mare, so the demon that visited people in the night came to be called a nightmare, along with "night hag" and "the riding of the witches."
The "lady-horse" mare also comes from Old English, but from mere; nightmares and equine mares aren't etymologically related.