Its not just regal tauntaun. I have 2 characters so far, one Jedi Consular and one Imp agent that on mounting any 2 legged creature turn into mutants. So far its 2 out of 3 characters. I think I'm afraid to look at the rest! I've tried tundra tauntaun, orobird, and kybuck.
oh dear. I dared look. 4 characters are freaks. All females. Wonder why my males are immune?!