Sounds like it could be scratches.
"Scratches is a dermatitis, or inflammation of the skin, and the most common cause seems to be the fungus Sporotrichum schenki. Some horses seem to be more susceptible than others, just as some seem more vulnerable to other fungal infections such as ringworm and girth itch. The fungus lives in organic matter and enters through breaks in the skin when the horse walks through contaminated pastures or muddy, swampy areas.
The dermatitis that results is basically an inflammation of the deeper layers of the skin, sometimes involving the blood and lymph vessels. The most common site of inflammation is the pastern and fetlock area, often in the heel and back of the pastern where the foot bends. The involved skin becomes warmer, reddish and thickened. Then the skin surface becomes scabby and cracked, and if the condition is not treated it usually becomes badly cracked and oozing and spreads to include larger areas. Infection may also spread to the inner tissues and is sometimes complicated by bacterial infection as well. The thickened skin may come off, leaving bare spots covered with rough skin, or raw areas.
Traditional treatments for scratches were astringents like methylene blue, iodine mixed with glycerine, or ointments made with zinc oxide, nitrofurazone and steroids. But a better treatment, recommended by several veterinarians, is a mix of nitrofurazone, DMSO and thiabendazole (a cattle wormer that is also a good fungicide)."
