I think its an odd position and worth digging at. If something's appearing in like, Fantasia and Narnia, to say nothing of other well known works, there's a point where we can say its a part of the general fantasy zeitgeist and could reasonably be encountered in any sort of generic fantasy setting, the likes of which most D&D settings are. I also think its important to the overall question, if you're going outside of human? You're going to roll things that have inspired from other media. LotR is obvious the big shot here, but likewise media with centaurs easily could come up as an option.
Wacko species to me sounds like "So atypical from your standard fantasy convention that you would not reasonably expect to encounter them", but centaurs are as in-built to this genre as elves and dwarves. Once again I'd bring up Gith as something that goes into wacko territory, because wrinkly humans who lay eggs is more a sci-fi type of thing