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She may have cullicoides(biting midges) sensitivity which is also known as summer eczema. This is the most liikely cause as sensitivity causes them to rub their mane and tail. They feed at dawn and dusk so it is best to keep them inside during these times. You can try hydrocortisone creams and sprays to help reduce the inflammation. Oral antihistamines will help with the inflammation and allergic reaction as well. Also keep her well coated in fly repellent. The sun breaks the fly spray down so get one with a sun screen in it. I would also keep a fly sheet, with a neck attachement, on her.

She could also have inhalant allergies which is called "atopy." She could be allergic to dust, molds, pollen, trees, grasses, pollen, and weeds. This is often seasonal and her symptoms may flare up with the change of seasons. The vet can now take a blood sample and send it off to see what she is allergic to. Then you can try to prevent it. You can also give her antihistamines to try and control the itching. Hydroxyzine is an antihistamine that works fairly well in most horses. All horse antihistamines are prescription.

She could also have mites. These are microscopic bugs that live in the hair follicles. The diagnosis is made with a skin scraping of the affected area. Treatment is ivermectin.

She may have ringworm. Ringworm is a fungal infection and not a worm at all. It is diagnosed by plucking hairs from the affected area and sending them out for culture. If the culture is positive, she will be treated with lyme sulfur dip , iodine, or miconazole to kill the fungus. Your vet may have a favorite treatment.