Thinking more on my last post, and more specifically about its context in my Wilderlands game, it occurred to me that the more likely reason that the presence of the Gnolls and Lizardfolk is tolerated is because where Bugbear Falls is located is pretty isolated, and the "monstrous" races have goods that the (primarily) human settlements need, as well as the might and intimidation factors to keep things cool when they come to town. Nobody wants to piss off a whole tribe of half-crazed hyena men that are rumored to worship demons, after all.
I think intimidation, and likely ignorance, is a powerful factor here. The human societies might not necessarily know much about their humanoid neighbors, but what they have heard - the stories that arise from twisted cautionary tales, or even second-hand tales of battles - color how they interact with them. So then, it becomes more a matter of fear. That's interesting to me for a couple of reasons...
Primarily because you might then see merchants give better prices to these "monsters" because they're afraid that charging normal prices might offend them, or they might get better quality goods or heartier portions. Tensions might increase, but never erupt toward the Gnolls or whatever race happens to be around, because "Their whole tribe will rise up!" or whatever other fearful reason might be given. This could make citizens hostile toward other citizens or the local government for inaction.
On the flip side, you have this Lizardman who doesn't know why he's getting pretty okay treatment. He might feel the tension...see the body language of the humans and know that they don't really like him around, but then be confused when his cut of pork is half again the size of everyone else's, or his shield seems sturdier. This could even lead to a sense of entitlement, which might then cause fractured relations with the humans once everyone is used to each other and the preferential treatment stops.
I find it all somewhat amusing, as social interactions are something that interest me, anyway. More to think on as I build my game further, at any rate, and more that I can share with anyone keeping up at home.