The Nightmare is a 1781 oil painting by Swiss artist Henry Fuseli. It shows a woman in deep sleep with her arms thrown below her, and with a demonic and ape-like incubus crouched on her chest. The painting's dreamlike and haunting erotic evocation of infatuation and obsession was a huge popular success.
After its debut at the 1782 Royal Academy of London, critics and patrons reacted with horrified fascination. The work became widely popular, to the extent that it was parodied in political satire and engraved versions were widely distributed. In response, Fuseli produced at least three other versions.
I always found four poster beds with the curtains to be a terrifying idea... the brain always imagines monsters poking through or hiding.