Sleep Paralysis Demon

The Sleep Paralysis Demon (as it is famously called) is a very unique type of Mare that appears during the process of Sleep Paralysis. It is the only type of dream entity that can briefly cross over into the real world.

Behavior
Sleep Paralysis incorrectly awakens you while most of your body is still in the process of REM Sleep, leaving you unable to move and suspended in a half-dreaming, half-awake state wherein dream-generated hallucinations can reside. Your brain, panicked and confused at the state it's in, will attempt to make sense of your paralysis by generating a Mare that is holding you down, whether it be physically or by it's own pure malicious energy. A common tactic of these Demons is to sit on the dreamer's chest, reacting to the shortness of breath caused by this condition. They may also strangle, injure, approach, speak to, or simply stand and stare as you lie helpless. These attacks can last anywhere from mere seconds to minutes.

Trivia

 * Many have and still do believe Sleep Paralysis to be attacks from paranormal beings.
 * The word "Nightmare" may owe itself to the Sleep Paralysis Demon, being based on the "Mare" from Old English folklore, a monster who sits on the chest of sleeping humans and causes them to have bad dreams, which aligns with the Sleep Paralysis accounts of the time.