There are different degrees of frightening dreams, but the most terrifying cause sensations of suffocation and paralysis.
Literature best describes the sleeper’s sensation in the stories of Dracula, but there was also a common female demon known as “the night hag.”
Mare is an Old English term for demon and comes from the same root as murder.
Therefore the demon, or mare, who visits at night was called a “nightmare.”
A bad dream is often referred to as a nightmare.