A Mare was believed to be a spectral visitor: a dark spirit that takes the form of an animal, a beautiful lover, a disfigured hag, or a demon which visited men in their dreams, and rode their chests, bringing on bad dreams or "nightmares," and dragging life out of them.
Aside from demons, Mares could also be witches, who were known to take on the form of an animal when their spirits went forth from their bodies in trance, called a “fetch”. These were usually small animals which left the body through the mouth, such as: moths, bees or wasps, birds, or frogs, and acted as a vessel for the disembodied soul, and as such was directly linked to the person’s fate and well-being. The fetch could also take the form of a larger animal, usually a hare, a cat or dog, a horse, or even an oxen. The fetch became corrupted when weaponized and projected forth for nefarious purposes, such as to cast malefic witchcraft or blight, or to drain the vitality of others, and becomes a foul hag or mare; akin to a projection of the ugliness of the person's true essence. Moths especially were associated with the Mare, and often finding them dead where one sleeps was believed to indicate her visitations.
The word "Mare" comes, through the Middle English mare, from Old English mære, mare, or mere. These in turn come from Common Germanic marōn, from which are derived the Dutch: "nachtmerrie”, and German: "nachtmahr”. The -mar in the French word “cauchemar,” meaning "nightmare,” is borrowed from the Germanic through the Old French mare. The word may ultimately be traced back to the reconstructed Proto-Indo-European root mer-, meaning "to rub away" or "to harm". Hungarian folklorist Éva Pócs endorses an alternate etymology, tracing the core term back to the Greek μόρος (Indo-European moros), meaning "death".
In Norwegian and Danish, the words for "nightmare" are mareritt and mareridt respectively, which can be directly translated as "mare-ride". The Icelandic word martröð has the same meaning: -tröð, from the verb troða, meaning "trample", "stamp on", related to "tread". Whereas the Swedish mardröm translates as "mare-dream".
The Mare was also believed to ride horses, which left them exhausted and covered in sweat by the morning, or sickened to the point of death. She could also entangle the hair of the sleeping man or beast, resulting in "marelocks," or “hag knots”. This is closely related to the lore surrounding Hag-Riding.
The German Folklorist Franz Felix Adalbert Kuhn records a Westphalian charm or prayer used to ward off mares, from Wilhelmsburg near Paderborn:
Hier leg' ich mich schlafen,
Keine Nachtmahr soll mich plagen,
Bis sie schwemmen alle Wasser,
Die auf Erden fließen,
Und tellet alle Sterne,
Die am Firmament erscheinen!
Here I am lying down to sleep;
No Night-Mare shall plague me
Until they have swum through all Waters
That flow upon the Earth,
And counted all Stars
That appear in the Firmament!