Lately I have been on yahoo answers answering peoples questions a lot like this. And it seems like the answer I can ALWAYS give them is "PAT PARELLI'S NATURAL HORSEMANSHIP 7 GAMES!!"
In this case the "friendly game" would do well for your horse.
Here is what I would do:
HOLD the horse with your hands, do not tie it up to something they can't get away from.
Start with something small such as a white piece of paper.
Begin by letting the horse sniff it. If the horse panics at the simple SIGHT of it. DO NOT DROP IT AND KEEP A FIRM GRIP ON THE HORSE.
Remain VERY calm and say "whoa" in a soft voice. Do not smile and do not look angry. Make sure your face remains blank.
Hold the paper down at your side and let the horse spook. Wait until the horse has become calm again and put the paper near the horses nose again to let it sniff the paper. When it accepts the paper near its nose, smile and tell the horse that it has done good, continue to tell it that it has done good while you RUB the paper on its nose, then on its cheeks, then neck. If the horse spooks at any one of these do the same thing, calmly tell it to "whoa" and put a blank look on your face. Put the paper to your side, but don't let go of the horse or the paper.
Then once the horse is calm again, start from the beginning.
Eventually, the paper to the nose will be a COMFORT zone and you will be able to resort back to it when the horse spooks at the paper on its side or something like that.
After you are able to rub the paper ANYWHERE on the horses body change objects. Anything white will work.
Obviously you can't just RIP the fence out of the ground. So just stick to lots of white objects, big and small. Continue these same steps. Eventually the horse will not mind white things anymore!
You can tell a gelding, you can ask a mare, but you must discuss it with a stallion.