incubus (n.) Look up incubus at Dictionary.com
"imaginary being or demon, credited with causing nightmares, and, in male form, consorting with women in their sleep," c. 1200, from Late Latin incubus (Augustine), from Latin incubo "nightmare, one who lies down on (the sleeper)," from incubare "to lie upon" (see incubate). Plural is incubi. Compare succubus.