First be sure that this is not a manners issue. Does your horse bite, kick or lay back its ears while being saddled, lead or during other handling? If it does, than you’ll need to work on respect and obedience.
If your horse is otherwise mannerly, consider that your horse may be sensitive or ‘thin skinned’. Horses are somewhat like people in their sensitivity. Some like a good vigorous massage, while others might find it too stimulating or even painful.
Often breeds like Arabians and Thoroughbreds are more ‘thin skinned’ than ponies or draft breeds. They don’t like a very hard brushing and you may have to lighten the pressure on the brush so they can tolerate it. I once had a Quarter Horse that preferred a light whisking off, rather than an energetic currying. My Draft Cross stood a good rigorous brushing with her lips twitching and her eyes half closed! It may be a matter of adjusting your touch to the horse’s sensitivity
Some horses have ticklish areas—usually along the belly, on the face and between the legs. They can get quite snappish if you brush these areas even lightly. You can easily desensitize your horse to make grooming pleasant for both of you.
To desensitize your horse, try only brushing with pressure it can accept in areas that don’t cause the horse to react. Little by little, push the boundaries. If the horse flinches and reacts negatively, retreat to a ‘safe area’ on its body. Work slowly and stay relaxed. Use your voice soothingly. Don’t try to get the whole horse covered in one day and don’t let it come to a test of wills. If a brush seems too harsh for your horse, start with a cloth or wear cotton gloves and just use your hands.
One method that works very well on a horse that dislikes grooming is TTouch. The Tteam website has several articles explaining TTouch and examples that you can try on your horse. Examples of Touch and How to Do Ttouchs explain the method clearly, so you can use your knew knowledge on your horse right away.
If your horse suddenly seems sensitive for no reason look for heat or swelling that may indicate an injury. The beginnings of rain scald or other skin problems, or a bruise from a kick may make your horse nip if the area is brushed. As well, mares might be flinchier at various times of their estrus cycle. If you suspect this, talk to your veterinarian about possible treatments.