My Paso Fino mare has them. Last yeara, a few small round white spots came out of nowhere. Winter came and went with no increase in the spots. Then, voila, many more spots appeared this summer. Seemed like one day, had the same number as last summer (only a few) and then, suddenly, she had twice as many.
The spots are not from ill-fitting tack because they are on her neck (under the mane) and on her rump and her saddle is specifically sized for her. They're not from fungus because they occurred in the summer when she had her summer coat and she was in a stall at the trainer's unless she was being worked. So those two possible causes of white hairs have been pretty much eliminated.
So, assuming that these are birdcatcher spots, (and I'm pretty sure that they are), at least this one Paso has them.