LOL! Poor fence design? There was nothing wrong with the fence. The fence was ripped apart as a result of gross negligence of another. And it really doesn't matter whose horse did the damage. Basic, fundamental common horse knowledge is that you shoo away horses from potentially hazardous...
So if you leave your house with a car that is in perfect condition and upon returning find there to be $4k in damage due to negligence of another person, who do you hold responsible?
We own 35 acres with horses. In order to protect our vehicles from the horses chewing on our vehicles we have four posts in the ground with a single strand of barbed wire attached to the posts. In order to get in/out we have a handle attached to the end of the barbed wire and simply loop it...