Night hag

The night hag or old hag is the name given to a supernatural creature, commonly associated with the phenomenon of sleep paralysis. It is a phenomenon during which a person feels a presence of a supernatural malevolent being which immobilizes the person as if sitting on their chest or the foot of their bed. The word "night-mare" or "nightmare" was used to describe this phenomenon before the word received its modern, more general meaning. Various cultures have various names for this phenomenon and/or supernatural character.
Sleep paralysis.
The original definition of sleep paralysis was codified by Samuel Johnson in his "A Dictionary of the English Language" as "nightmare", a term that evolved into our modern definition. Such sleep paralysis was widely considered the work of demons, and more specifically incubi, which were thought to sit on the chests of sleepers. In Old English the name for these beings was "mare" or "mære" (from a proto-Germanic "*marōn", cf. Old Norse "mara"), hence comes the "mare" part in "nightmare". The word might be etymologically cognate to Greek "Marōn" (in the Odyssey) and Sanskrit "Māra".