/mɛər/
[noun] An adult female horse. The word can also refer to a female donkey or mule in some contexts.
The 'mare' in 'nightmare' has nothing to do with horses — it comes from an evil spirit believed to sit on sleeping people's chests.
The word 'mare' for a female horse is one of the oldest words in the English language, tracing its roots back to the Proto-Germanic word *marhijō, meaning simply a female horse. This ancient form is closely related to the Old English word mearh, meaning horse, which itself descended from Proto-Indo-European *marko-, a root that linguists believe referred to horses in the ancient world long before written records began. The word passed through Old English largely unchanged in meaning, solidifying its place in the language as the standard term for an adult female horse.
Across the Germanic family of languages, this root gave rise to a rich collection of related words. Old High German had marah for horse, and the compound word marahscalc — literally 'horse servant' — described the person in charge of horses in a noble household. This compound traveled through Old French as mareschal and into Middle English as marshal, giving us the modern word 'marshal', which began as a job title for a stable keeper and rose in prestige over centuries to mean a high military officer. The chain runs: Proto-Germanic *marhaz → Old High German marah → Old High German marahscalc → Old French mareschal → Middle English marshal → English marshal.
The word mare in its equine sense has remained remarkably stable in English, unlike many of its relatives. While 'marshal' climbed the social ladder and shed its stable-yard origins, 'mare' stayed close to the paddock. It appears in Old English texts from before the Norman Conquest and has been recorded continuously in written English ever since, making it a genuine linguistic survivor. Writers from Chaucer onwards used it freely, and its meaning has never seriously drifted from its original sense.
However, English also contains a completely different 'mare' that looks identical but comes from an entirely separate root. This is the 'mare' in the word 'nightmare', and it has nothing to do with horses. This mare descends from Old English mære, which in turn came from Proto-Germanic *marōn, referring to a type of evil spirit or goblin believed to visit people while they slept. The creature was thought to crouch on a sleeper's chest, causing a crushing sensation and terrible dreams. Old Norse had a similar word, mara, describing the same demonic being, and related forms appear in German as Mahr and in Old Irish as mor. The nightmare, then, is literally the 'night spirit' — a terrifying visitor from the dark, not a frightened horse galloping through your dreams.
This double life of the word 'mare' — as both a real animal and a ghostly demon — is a beautiful accident of language, a collision of two entirely different words that happened to look the same after centuries of sound change. Astronomers added a third layer of meaning when they named the large dark flat plains on the Moon maria, using the Latin word for sea, because early observers thought these shadowy regions might be bodies of water. Though this Latin mare is unrelated to either the horse or the nightmare spirit, the three meanings now share one small word, making 'mare' one of the more quietly crowded words in English.