If I remember right, there was someone making a giantess monster girl fighting game that browses 8ch, though not every giantess is a different mamono (there was an army loli that's a muscleloli under her clothes, there was 2nd pic related, etc.) The first pic is also a playable character.
However, for a /monster/ fighting game, what I would want is an overly complicated rhythm-based fighting game that could only possibly work with a custom arcade control 88-key keyboard design. Monster girls from all over the globe would represent stereotypes of their location and use instruments related to their area of origin for magical attacks related to what they specifically are as a monster girl. This would also present the opportunity to make a lot of OC monster girls based on monsters that haven't had monster girl content on them yet. For example, the undead "draugr"s of Scandinavian lore are described as "that of a dead body: swollen, blackened and generally hideous to look at", so in a sly jab at Sweden, a Swedish draugr could play into both the older stereotypes and more modern stereotypes of Sweden by giving us a busty blonde draugr girl with skin blacker than a hellhound. Ammit could serve as the friendly final challenge of arcade mode and use these as her borrowed instrument-weapons: https://en.wikipedia.org/wiki/Tutankhamun%27s_trumpets . Other ideas for characters include but are not limited to:
<New Orleans golliwog/living voodou doll with a banjo
<extremely patriotic sandworm with a kazoo (Washington state is where Dune was written)
<a southern Italian faun too lazy to learn an actual instrument so she uses wine glasses instead
<sewer gator girl from New York with a bass guitar
<French Tarasque (not the Tarrasque from DnD, the french monster Bowser's design was based on) with a lazer harp
<Scottish Lavellan (https://en.wikipedia.org/wiki/Lavellan) girl with bagpipes
<South African Grootslang (https://en.wikipedia.org/wiki/Grootslang) girl with vuvuzela
<Papua New Guinea ropen (http://cryptidz.wikia.com/wiki/Ropen) girl with a thongaphone (https://en.wikipedia.org/wiki/Thongophone)