Not OP, but monster girls are typically fantasy creatures while furry are amalgamations of real animals and grills.
David Ross
I see what you mean, but as someone who really likes monster girls and has played a lot of games such has MGQ, I noticed a lot of the monster girls are based on real animals/plants/insects/fungi/demons etc. Fox girls, snake girls, earthworm girls, mushroom girls, dragon girls, etc. I would see where you're coming from, I would argue that there is a fine line between animal based monster girls, and furry girls. The main difference I see would probably be that furry girls tend to be more, well, furry, as well as some tendencies in art-style difference.
Those incorporating real species, I'd say benign species are more 'furry'; while the more aggressive and predatory species are more 'monster'. >Cat girls are furry, but dragon girls are monster
Its super simple. Monster girls generally have a human face and nose. Furrys have an animal face and nose, usually a muzzle. They also generally have fur on their actual face.