>>13645545Guys into monstergirls imagine them as having simpler personalities like a domesticated animal but with human features, like wolfgirls that like to be petted or catgirls that like to curl up on laps. They are also usually highly expressive with their animal features (wagging tail, wiggling ears, etc) which is a trait animals have that people find cute.
It's similar to why there are anime tropes about girls with specific personalities that are highly focused on a man (even tsundere, yandere, etc have this trait even if they're mean to that guy), it's easier to imagine that that character is placing the viewer as the object of their attraction.