The bitter, bitter war between MGs and Anthros is a fact of life around here. However, both fetishes ultimately tap into the same xenophiliac base roots. Consequently, the line can get blurrier more than people care to admit.
So, just how and where do you draw the line? What traits do you call on to distinguish between a monstergirl and an anthro?
For example: this picture is G.I.N.A: Godzilla Inspired Naked Anthromorph. Once the mascot of Daikaiju Academy, a web-site subsection dedicated to profiles & artwork for sexy kaiju-based monstergirls, until the web-site's owner threw a hissy fit that more people were coming for her and her friends than for his shitty webcomic.
(No, it's not one of the best pictures of her, but it's the only one of the original artworks by her creator featuring her I can find now.)
Anyway, as you can see, her face is humanoid (though some of her friends aren't quite so human-faced, depending on the writer). But the rest of her body isn't. She's got no "distinguishing lines" between the scaly parts of her body, like one of KC's lizardgirls. Even her creator calls her an "anthromorph". So, does she count as a monstergirl?
Or, for a still-around example, look at Flora, tiger-girl female protagonist of the webcomic "TwoKinds". Her companion Natani is clearly an anthro, complete with snout. Flora, though, has a flat, anime-style face that is more "human with feline traits" than "expressive muzzle". She's got fur all over her body, paws for feet, and walks upright on digitigrade legs. Is she an MG or an anthro?
Heck, I'll even get a little personal here; a design for a hydra-girl I've been cooking up starts with a human girl as a basis. It gives her digitigrade, reptilian-pawed legs, claws on her fingers, an elongated and prehensile serpentine tongue, slit pupils, scaly skin, draconic horns, hair, and her "extra heads"; from her back, like a mockery of wings, she has scaly tentacles, each tipped with the small head of an anthro snake bearing draconic horns. Another, larger such tentacle-maw emerges from the base of her spine and functions as a tailmaw. Is she an MG or an anthro?