?Cover? Rubbing large areas of my horse's hair out. Help needed please!?

I'm going to chime in on thinking this may be an internal issue. Many metabolic disturbances manifest with skin conditions. I would not buy any more rugs until you have your horse's skin tested and blood work drawn to check the chemistry and counts. This way you can at least rule out medical issues that could be causing the skin reactions. If a metabolic issue or skin condition shows up, then hopefully treatment will resolve the problem.