Nightmare looks like it should mean a bad dream involving a horse, but that is not where the word comes from. In this word, mare is an old Germanic word for a harmful spirit or demon believed to trouble sleepers.
Long ago, people used related words in several languages for a being that seemed to sit on a sleeping person and cause fear, pressure, or terrifying dreams. Over time, nightmare became the normal English word for a very frightening dream.
This is a good example of two words that look the same but have different histories:
- mare in nightmare: an old spirit linked with bad dreams
- mare meaning horse: an adult female horse
So when someone says, “I had a nightmare,” the word has nothing to do with horses. It is about the older sense of a night terror. The matching spelling is just a coincidence in modern English.