This is from a book about Mary Shelley (title and author not noted, unfortunately) and how she came to create the story of Frankenstein. It says Mary would have been familiar with the painting shown here, The Nightmare by John Henry Fuseli. So the book says,
the impish creature is an incubus, once believed to cause nightmares by sitting on the sleeper’s chest…But the creature is not on the woman’s chest. Her chest is facing us, quite unobstructed by anything. He sits on her abdomen, and if the imp is the kind with a tail, his tail is you-know-where.
Incidentally, here’s a line from the Wikipedia entry, explaining why Fuseli is not considered much of a colorist.
He scorned to set a palette as most artists do; he merely dashed his tints recklessly over it. Not infrequently he used his paints in the form of a dry powder, which he hastily combined on the end of his brush with oil, or turpentine, or gold size, regardless of the quantity, and depending for accident on the general effect.The Wiki writer calls this “recklessness” and goes on to say,
Despite these drawbacks he possessed the elements of a great painter.What recklessness? What drawbacks? Seems to me Fuseli was only ahead of his time. Plenty of painters take a slapdash approach to what they throw on the canvas. Accident is what the whole art of raku pottery is about. It’s an element of technique, not a pathology.