nightmare: [13] The mare of nightmare is not the same word as mare ‘female horse’. It comes from Old English mære, which denoted a sort of evil spirit or goblin which sat on sleepers’ chests and gave them bad dreams. That is what the compound nightmare meant too when it emerged in the early Middle English period, and the metaphorical application to the bad dream supposedly caused by this incubus is not recorded until the mid-16th century. => yell[nightmare etymology, nightmare origin, 英语词源]
late 13c., "an evil female spirit afflicting sleepers with a feeling of suffocation," compounded from night + mare (3) "goblin that causes nightmares, incubus." Meaning shifted mid-16c. from the incubus to the suffocating sensation it causes. Sense of "any bad dream" first recorded 1829; that of "very distressing experience" is from 1831. Cognate with Middle Dutch nachtmare, German Nachtmahr.