Spending at least 4 hours per day in front of baboon enclosures when I was observing their grooming behavior means that I get to eavesdrop on what zoo goers have to say when they first see a baboon. The kids (who doesn't really hold back on anything) would usually yell out loud, much to the embarrassment of their parents "OMG look at that ugly red butt". I've been asked many times (they think I work for the zoo) why baboons have red behinds and why do some looks like red tumorous growth.Whenever someone sees a baboon, their immediate reaction is to comment on the butt. Always.
So, I was pretty surprised to learn that there is dermatitis problem call "baboon syndrome" in humans. You guessed right, the baboon syndrome is an allergic reaction that causes the butt to turn red! Neurotopia has a great write-up of what the baboon syndrome is, "Friday Weird Science: Redder than a baboon's butt".