There is no horse in a nightmare. The mare is Old English mære, a spirit that was believed to sit on a sleeper’s chest and press the breath out of them, and it has nothing to do with the mare in a field, whose name comes down a different road. In 1781 Henry Fuseli painted The Nightmare with the right creature squatting on the sleeping woman and, behind the curtain, a horse’s head looking in, probably as a joke. The joke has worn better than the word. Ask most people now what the mare in nightmare is, and they will tell you it’s the horse, the one that only came in to look.