Interesting isn't it how imagination blends different elements across language and culture? The English term "Nightmare" whilst obviously conjuring horses to mind, is actually from the Norse word "Mara" (Old English mære) meaning "ghost" A Mara was a believed to be a being that infested dreams, when people lacked the modern notion of dreams being an active part of the brain's function.
Not mentioning that your a wonderful skilled artist, but this is totally differenct to Fussli's Nightmare, but you already said this is modest compared to his work. I really like the conception, even there isn't much eroticism in it, but instead of that there is an extreme spiritual feeling in it. Probably because of your "dead" horse and the pale light. I think Fussli did a majestic work with "The Nightmare", but he would have done even better with your design for the imp, while you would have done better with his version of the horny horse. xP All together I have to say it's a very beautiful piece of work, good job!
Wow - I just recently came across the "Nightmare" painting that you reference and thought it really fascinating and I think what you have produced here yourself is truly spectacular - so delicate and atmospheric, it's a real masterpiece! Thank you so much for sharing.
Overall I love your interpretation a lot. I know that this is your own interpretation but one discrepancy which bothers me is that the imp is not sitting on the woman's chest, which changes the meaning. By placing the imp on the chest, Fuseli was alluding to sleep paralysis. Other than this minor detail it is really well done!