A mare (Old English: mære; Danish, Norwegian: mare; Swedish: mara; Polish: mara, zmora; among many others) is a malicious entity in Germanic and Slavic folklore that sits, walks, or “rides” on people’s chests while they sleep, bringing on nightmares.[1]
The mare was believed to ride horses, which left them exhausted and covered in sweat by the morning.
(this reply is related to the painting the meme is based on)
https://en.wikipedia.org/wiki/Mare_(folklore)