No, a nightmare is not a dream about a scary horse. The origin is straightforward, but not obvious to the modern ear. The first syllable is easy, it is the same as the word night. It’s the mare part that makes people think that nightmares have to do with horses.

The word nightmare makes its appearance c.1300 in the St. Michael (Laud) manuscript:

Þe luþere gostes...deriez men in heore slep...And ofte huy ouer-liggez, and men cleopiet þe niȝt-mare.
(The wicked spirits...injured men in their sleep...And often lay on top of men, and men called them the nightmare.)

Mare is simply an Old English term for a demon that suffocates one in one’s sleep, an incubus or succubus. So a nightmare is a demon that visits you at night—a scary dream. Over the centuries the meaning has become generalized to any frightening dream. A similar term for the original phenomenon, still used today, is night hag.

As for its components, night is recorded in the Vespasian Psalter from c. 825, but it is such a basic word that it is likely far older than surviving manuscripts. It has cognates in most of the Indo-European languages and is from the Indo-European root nek, meaning dark or night.

Mare is found in the Corpus Glossary, c.725:

Incuba, maere.

Mare also has cognates in many languages and ultimately derives from the Indo-European root mer-, meaning to to harm, and which is also the root of murder and mortal.

The psychological phenomenon of night hag is well documented. Some people experience a sensation of paralysis, including difficulty breathing, and couple it with a sensation of being visited by other beings. In today’s modern mythos, it is usually extraterrestrials, not demons, that visit and perform strange sexual acts. The paralysis and visitations are not real, just figments of the mind in active dreaming. But these dreams are powerful, as attested to by the persistence of the myth, only the supposed origin of the imagined visitors has modernized.

(Sources: Oxford English Dictionary, 3rd Edition; American Heritage Dictionary, 4th Edition)

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