If you have ever pictured a literal female horse galloping through the imagery of a "nightmare," the etymology has a small surprise in store. The "mare" in nightmare is not a horse at all. It is the ghost of an Old English word, mære, that named a particular kind of evil spirit — one that was believed to sit on a sleeper's chest and bring on terrifying dreams.
An Old Germanic Demon
The Old English mære belongs to a family of related Germanic words: Old Norse mara, Old High German mara, modern German Mahr, modern Swedish mara. All of them denoted a supernatural being — usually female — that pressed down on its victims at night, causing the heavy, breathless feeling we now recognise as sleep paralysis. The condition is real and well-documented in modern sleep medicine: a brief period after waking when the body remains immobilised and the mind, half in REM sleep, conjures vivid, often menacing imagery.
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From Spirit to Bad Dream
The compound "night-mare" is first recorded in English around the 13th century. Its earliest meaning was the creature, not the dream — a "nightmare" was a goblin you might encounter while asleep. By the 16th and 17th centuries, the word was drifting: people began to use it for the experience the creature was thought to cause — a suffocating, terrifying dream — rather than the creature itself.
By the 19th century, the supernatural sense had largely faded. A "nightmare" was simply a frightening dream, no spirit required. The metaphorical leap to "any extremely difficult or unpleasant experience" — "the commute was a nightmare" — followed easily from there.
Henry Fuseli's Painting
A Picture of the Old Belief
The Swiss painter Henry Fuseli's 1781 work The Nightmare shows a sleeping woman draped across a bed, with a small grinning demon (the mære) crouched on her chest. Behind them, a wild-eyed horse pokes its head through a curtain. Fuseli was leaning into the visual pun — the horse is there because, by his time, the older meaning of "mare" was already being lost. The painting is a snapshot of a word in transition.
The Other "Mare"
The horse-mare in modern English — an adult female horse — has its own, completely separate origin: Old English mere, the feminine form of mearh, "horse." The two words look identical today only because of centuries of sound change. They are linguistic homophones, not relatives. So a "nightmare" and a "mare" are, despite appearances, etymologically unrelated.
Cousins Across Languages
The old mare-spirit has left fingerprints on several other modern words. French cauchemar ("nightmare") preserves the same root in its second half — the first part comes from a verb meaning "to trample." Old French mare faded from general use but survived in this compound. Dutch nachtmerrie uses the same Germanic root, as does Norwegian mareritt ("mare-ride").
And the verb to be hag-ridden, which today means to be tormented by anxiety, originally referred to exactly the same phenomenon: a hag — another name for the mære — "riding" a sleeper through the night.