I feel your pain, although not with horses.
Our yellow lab is Houdini incarnate. Last winter he would climb the fence when ever he would see my husband. He'd climb it and come to the front deck and look in our living room windows.
We tried digging around the fence to make 'lower' for him, we tried staking him out, and waiting for him to climb it, and then shock the crap outta him with the shock collar, AND we tried staking down the bottom of the fence as well.
Finally I had enough! I ORDERED the ole man to fleet farm to get some hot wire. He wired up the dog's fenced in yard in the dead of winter
No more than 5 mins after he had come in after finishing the last touches we hear a :
I'm sorry, but I was so frustrated with that dog, I was just a bit satisfied when I heard him yelp. So, now, we dont need to keep the fence on all of the time, and if he's going to escape, he's going to do it a different way, he knows better than to go over the top.