The two major potential diagnoses for your mare are sunburn/photosensitization (not likely since you state that it has been going on for some time and only the genital area is affected) and a possible skin cancer. The most likely skin cancer to rule out is squamous cell carcinoma.
The first step to determine the cause is to have your veterinarian do a complete examination of the mare and take a biopsy of the affected area for histopathology. Treatment of the condition will be based on the diagnosis and the amount of her genital area that is affected.
