Yes, if you are grooming your horse with harsh brush and excessive force, overgrooming is possible. Even more if you are applying force on the sensitive areas. Overgrooming can leave a horse with sore skin and hair loss, and even result in infection. You should pay special attention while grooming a horse when he is ill or injured. It's best to leave a horse alone when he is in pain. If your horse is in pain, you can massage him with a soft brush to ease the pain.