Originally posted on 2chan/Futaba back in 2006, at the genesis of monstergirls' codification and rise to popularity. Though they had existed for a long time before this, it was there that the term "monster musume" (monster girl) was coined as a specific genre. (or fetish, if you're more cynical; my first encounter with them was admittedly porn)

This chart is essentially a summary of what everyone in the threads agreed constituted a monster girl.

(Translations mostly cribbed from Danbooru.)