I is for Igor... which raises the question: is Igor really a monster? He's a deformed human, which suggests that he's not a monster, he's a human. But he does perform some monstrous acts, such as grave robbing and stealing body parts, which makes him a sort of symbolic monster. In any event, he's a mainstay in monster culture, being present in Universal horror movies, knockoffs, and spoofs for years and years.
He's a hunchbacked lab assistant/butler, but is he such a bad guy? Maybe he is, or maybe he's just a misunderstood weirdo. In any event, this character has earned a place in my "Monster Alphabet" series.