Succubus & incubus don't show up in the Latin dictionaries I've searched. I'm wondering what the plurals would be. I did find succuba, 1st decl fem. Could it be that it didn't morph into a 2nd decl look-alike until it came into English? Or could it be an unusual 4th declension?
According to Wiktionary, succubus evolved in Middle English from the Latin word succuba, one who lies under. The plural is given in the aforementioned Wiktionary as succubi or succubuses. Personally, I've usually only seen the former used (or maybe I just prefer that plural).