Could be a range of things, moulting, fly bites, horse rubbed on a fence etc, but unlikely to be anything serious if there is no broken skin under the patches of hair and no other symptoms. I have often seen patches like this on horses and so far it has never been a problem and doesn't last long. If it got a lot worse (many patches) and the skin looked irritated then my first suspicion would be an allergic reaction or insect bites, both of which can be dealt with.
Thanks so much all, I don’t feel any abnormalities under the hair and she isn’t quite done moulting yet so probably that, I can always count on the forum!