With large horses, I don't think it works, but for ponies and quiet geldings and mares, it should be fine. Corner and gate posts are crucial to the strength of the fence, and should be larger and set deeper than the others. Frederick Harper, PhD, University of Tennessee, Knoxville, says particular attention should be paid to tamping the dirt with gravel and soil tightly around the entire depth of the post to increase stability. Some opt to add dry concrete mix to the hole, although that can create tension at ground level, increasing the risk of breakage. Dirt, stone, or concrete also can be heaped up around the post an inch or so above ground level in order to shed water away from the post.
Gates are an equally important element of a fence, again, constituting a possible weak link.
Make sure the gate is wide enough for your needs. Make sure there isn't a big opening between the top two rails where a horse could get a head in there. Wytiaz suggests installing the gate so it never sits on the ground. Horses are always milling at the gate, and in rainy weather, they really churn up a lot of mud. If your gate isn't high enough, it'll rust on the bottom and you'll lose your bottom bar.