A friend of mine has that problem with her clyde x, who looks more like an overgrown pitbull on steroids.
She has gone over the fences, so they made them higher... Then she would just bulldoze through them, so she had a strand of electric wire tied to her chest, after she got shocked from that she stopped going through the fences.... And started going under them... So now we have to find a solution for that.
I hope your horse isnt like that one!! It's a right pain
R.I.P ~ Bubbles - 25yo tb mare - 13.04.2011 ~ 8:30am ~ passed away naturally and peacefully in my arms