As I often do when I’m beginning a new essay, I seek out the etymology of the word I want to explore. This time, I’m disappointed that the word “nightmare” has nothing to do with horses. The idea of bad dreams as horses galloping through our night-time hours resonates with me. But mare comes from the old English maere, the word for a goblin who sits on a sleeper’s chest – a succubus. The Nightmare, the famous 1781 painting by Swiss painter Henry Fuseli, hedges its bets, depicting a squat gargoyle-like figure crouched on a swooning maiden, with a donkey-ish...