Marebus

Marebus,
A Succubus or incubus that mated with a Mare.

Succubus: A demon that commits sexual acts with you in your sleep and is a female
Incubus: A demon that commits sexual acts with you in your sleep and is a male
Mare: A demon that sits on you while you sleep and gives you nightmares

So, a Marebus is a demon that commits sexual acts with you while you sleep and gives you nightmares aswell.

Jessica: hey bob a marebus just fricked me last night as i had a nightmare
Bob: what the hell is that
Jessica: a demon that fricks you in your sleep and gives you nightmares :)