An adult female horse.
"The vast mare stretches across the moon's surface like a dark ocean under Earth's light."
A type of evil spirit formerly thought to sit on the chest of a sleeping person; also, the feeling of suffocation felt during sleep, attributed to such a spirit.
A large, dark plain, which may have the appearance of a sea.
In plain English: A mare is an adult female horse that has already given birth to at least one foal.
"The mare gave birth to her first foal in spring."
Usage: Use "mare" only when referring specifically to the frightening dream image or sensation of being unable to breathe while sleeping. Do not use this word for any other type of nightmare or bad dream that does not involve suffocation.
The word "mare" comes from Old English, where it originally meant a female horse. It traces its roots back to Proto-Germanic and earlier forms related to the general concept of a horse.