The modern word "succubus" wasn't a word in Latin and only arose during the medieval period due to influence from "incubus". The "-us" ending marks masculine nouns. This means that, arguably, "succubi" isn't a correct plural, because really it should be "succuba", plural "succubae".
Bonus fact: succubus comes from sub "below" + cubare "to lie down", while incubus comes from in "on" + cubare
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