This is quite interesting and in Scandinavia in Northern Europe, where I come from this is called a "Mara" (our word for nightmare is actually
"mardröm", which would be "mare-dream" in English.
The following I have taken from the Swedish Wikipedia and I have taken the liberty to translate some of it for your benefit, since I gather not many
people is fluent in Swedish on this board.
Any strange english wording then is probably caused by me...
"...Mara (unknown origin, Danish: "mare", German: "mahr" or more common "alp" or "alpdrücken", English: "nightmare", French "cauchemar",
Italian "incubo, Spanish "pesadilla", Latin "i’ncubus, asthma nocturnum", Greek "efialtes") was at least in earlier times a desciption for a
feeling of suffocation or a pressing weight on the chest that comes during sleeping. This often lead to a sudden and violent awakening with a feeling
of anxiety and worry.
The reason behind this can probably be manyfold, sometimes as severe as certain heart-conditions and digestion disorders, but sometimes this could be
stressrelated and was seen at least in earlier times for what was called neurasteni. [My comment: This seems to be a somatic heartproblem with some
symtoms common with what is described above]
Of the term "mara" the folklore created very early a sort of being or creature, that is mentioned already in the ancient Icelandic epic sagas and
was called the "Nattmaran" [My comment: Translated to English this is actually "Nightmare"]
In the Nordic region one thought of the "mara" as in the form of a woman, that sneak into the bedroom in the night, through the keyhole or the
slighest crack in the door or windows. The creature then sits on the chest of the sleeping person. This process was called "to be ridden by the
mare". [My comment: According to the folklore, the person ridden, woke up in the morning totally wasted and really tired]
Sometimes people said that this was in fact Lilit which was [My comment: in the Nordic folklore at least!] the mother of the "Skogsrå", [My
comment: this creature doesnt really have an equivalent in other parts of the world that I know of except in Japan. The "Skogsrå" was a sort of
Queen of the forrest and ruled over the animals. She was known to be very hot and sexy viewed from the frontside and was said to seduce men that was
out working in the forrest. However, if you saw the "skogsrå" from behind she had a hollow back and a foxtail]
Sometimes oridnary women could be unwillingly and unknowingly be temporarily transformed to "mares" during the night and thus haunt men in their
sleep. [My comment: I would think that this was to explain mens wet dreams about certain women, where they could claim that they had been ridden by
the woman in question and thus it wasnt "their fault" of dreaming sex-related dreams. But that is just my opinion...]
It was also said that the "mara" could not climb into bed to the sleeping person without first stepping into his shoes - that usually was placed
just beside the bed since the house and floor were usually cold in the morning. To counter this the folklore stated that the shoes should be placed
*under* the bed with the shoes pointing outward. [My comment: To make it pretty hard for the "mara" to get into the shoes. Why the "mara" couldnt
bend down and just get the shoes out doesnt folklore tell you...]
The "mara" was also said to be riding the horses in the stable and in the morning you would find the horses manes in a disorder..."