Just as you would with a strange dog, you should first ask permission before petting a horse. As prey animals, horses are afraid of new people. For all the horse knows, you could be a predator who wants to make a meal out of him. Learn how best to approach and pet a horse so that you will both enjoy the experience.
Saying Hi From Afar
Announce your arrival using a firm yet soothing voice or whistle, not by running up to the horse and screaming with excitement. A horse cannot see directly in front of his face or behind him. Approaching from these areas may cause him to rear up or kick. Let him know you mean no harm by walking towards his shoulder where he can see you.
A Calm Presence
The horse will pick up on your physical communication. Relax and refrain from fidgeting to show a calm exterior. When horses approach each other, they say "hello" by touching noses and sniffing. You can accomplish this by showing the horse your hand, just below his nose and allow him to investigate. Keep your fingers together so that he doesn't mistake them for delicious carrots. He is showing acceptance if he lowers his head, turns towards you or licks his lips.
As tempting as it is, don't pet the horse's head. Continue to face his shoulder from the side, petting the neck. Most horses enjoy a scratch on the neck just behind the ear or on the withers at the highest point of the shoulder. Discontinue petting if the horse backs away or shows disinterest. Vacate the horse’s personal space of 12 to 15 feet if he rears or kicks.