The work was likely inspired by the waking dreams experienced by Fuseli and his contemporaries, who found that these experiences related to folkloric beliefs like the Germanic tales about demons and witches that possessed people who slept alone.
Henry Fuseli – The Nightmare
The Nightmare was likely inspired by an interpretation of dreams based on Germanic folklore, in which demons possessed people who slept alone. In these stories men were visited by horses, and women were ravished by the devil. The woman is surmounted by an incubus; a mythological demon who lies upon sleeping women. It has remained Fuseli's best-known work. With its first exhibition in 1782 at the Royal Academy of London, the image became famous. After that Fuseli painted at least three versions.
To see the original: https://bit.ly/2VREsHx
Henry Fuseli - The Nightmare - 1871
Detroit Institute of Art