I agree with Vidalco - It sounds like scarring. If it is in her saddle area, it might have been caused by a badly fitting saddle. The saddle could have rubbed until it caused damage and the hair grew back white. The sporatic hairs could be roaning. Our eight year old chesnut mare (QH) has a very faint sprinkling of white hairs in her barrel area.
If you have papers on order any way, and you already own the horse, you might want to wait on the vet fees and just see what you learn when the papers arrive.