I think it's genetic. Some of the finer bred horses, perhaps thoroughbreds, don't grow so much coat. But that doesn't explain Cyn, who is Arab/Mustang not growing one.
Most of the Arabs I've known have grown pretty thick coats in winter, but many QH's... particularly those Appendix QH's with more TB in them don't so much.
Well Rowdy isn't appendix bred, but that doesn't necessairly mean he doesn't have TB in him.
The Arabs we use to have and all the other QH's we've had grow thick winter coats. Sometimes I wish they didn't grow so much:), it's really a mess in the spring.
Becky