Some horses are just more wary by nature than others and interpret any advancement toward them as a threat. After a while, it can become a nuisance at the least and possibly dangerous. If your horse tends to be jumpy, distrustful or resistant in other areas, too, it's worth the effort to get this horse to trust you.
Try to determine if the horse simply has a ?push my luck and see what happens? personality. Many horses with new owners/riders/handlers will try this game, especially if it's worked in the past and allowed him to successfully avoid being handled and worked. If the horse has days when He's fine and days when he acts head shy, odds are He's testing you.
it's also certainly possible he was never correctly trained and doesn't know any better. Either way, the solution is to go back to the basics.
Depending upon the degree of severity, you may have to begin with simply stroking the horse's head (avoiding the ears). After rubbing him a bit, extend your hand away from him, waiting for a slight head drop (indicating acceptance), then reward him.
Once He's quiet about that, proceed to using a towel, offering a face rub. Most horses love this, and once he figures it out, he'll drop his head willingly. You can eventually work your way up the rest of his face, toward his ear, with rubs. This will take time and patience.
Once the horse consistently relaxes through the poll and stops trying to escape your hand, you can proceed with haltering or bridling. Place your left hand across his nose and keep it there, halter or bridle in your right hand. Use a firm but calm tone and talk to him. Most horses give up when they realize you won?t be easily bullied.
Remember not to inadvertently cause that jerk-away reaction. Horses have a blind spot in their visual field directly in front of them. So, some horses may naturally become unsettled when you walk up in front of them because they can't clearly see what you're doing,
Without realizing it, you may also be reacting to his reactions, i.e. the horse startles and jumps back or puts his head up, so you also jump back or put your hands up. Your reactions fuel his and vice versa.
it's a different story when your horse is perfectly behaved in every other way or was never head shy in the past but develops this new problem. If He's easy to work with otherwise but has been like this as long as you have known him, poor handling during the breaking stages may still be the problem so backing up to desensitize/reschool him is still worth a try, starting with the rub rag. If that doesn't work, you may need to look for clues that indicate specific areas that could be causing the problems.
When a horse suddenly becomes headshy about haltering or bridling, a physical problem may be the reason. If he acts up before you even touch him, a change in vision is likely. Otherwise, look for pain at the poll, ears, jaw and nasal bones. If the problem is worse with bridling consider the teeth. If you pay careful attention, the horse will give you clues:
1) If pressure on the poll from a lead rope, or with your fingers makes him flinch or pull back, and he is resistant to flexing when working, suspect poll pain.
2) If the horse is fine until you try to manipulate his ears, suspect ear pain.
3) If he fights bridling more than haltering, suspect mouth pain, especially if He's also resistant to the bit when working.
you'll likely need a vet exam to determine exactly what the problem is, but you can save your vet some time if you?ve paid attention to details.
Horse Journal Staff article.