The first of these words appears frequently, the latter rarely. A succubus was originally an imaginary demon in female form that descended upon sleeping men and had intercourse with them. Similarly, an incubus was an evil male spirit that descended upon women. By extension, incubus (but not succubus) has come to mean something that weighs upon one's mind or anything that oppresses. A nightmare is a kind of incubus, but the word means any kind of burden or obstacle: "His mortgage was an incubus that he could not shake." "Knowledge of his guilt weighed upon the murderer like an incubus."
Dictionary of problem words and expressions. Harry Shaw. 1975.