No one is startled by a raven!
(Unless he has seen to many Hitchcock movies)
Did I mention the hybrid form where I'm a 5 foot anthropomorphic raven?

And the fact that there's an entire group of them in CoS? Never minding the freaking ANGEL hanging out in one of the nearby buildings. (sorry, don't want to spoiler too much) And the fact that the Mists kidnap all sorts of beings from the multiverse all the time, meaning that the folks in Barovia likely have seen all sorts of weird crap walking around.
This is, of course, ignoring the curse on Barovia where the population isn't exactly bog standard human anymore either.
But a walking lizard? Oh, man, that's too far.
Same goes for Mystara - Nostalgia feel? Sure, but, that's going to vary pretty wildly depending on when you played Mystara. One person's totally unbelievable addition is another person's first experience with the setting. I mean, take Diaboli. That's from the Poor Wizard's Almanac, 1992. That's a LONG way into the development of the setting. Yet, extra-dimensional travelers coming to settle in Mystara are perfectly fine, but, Tiefling's aren't?

Methinks the Lady doth Protest too much.