As free-ranging herbivores, horses have an instinctive phobia about being confined, and even several thousand years of domestication are not likely to convince them that cougars do not lurk behind every rock and tree. Color is not a key factor in visibility, but brightness and contrast are. Because a horse's eyes are attracted by movement, you can make a fence more visible by attaching strips of cloth or plastic at intervals along its length. These little flags, which flutter in the breeze, are often employed by those who have wire or electric fence, to call attention to the location of the top wire, and they can be very effective. Where you place your fence also has a lot to do with how visible it will be to your animals.
Horses are far-sighted creatures who tend to look to the horizon.
For maximum visibility, install your fence at a height at least equal to the height of your tallest horse's eyes, when he holds his head up. Most fence installers recommend a fence at least six feet high for stallions. Social pressures are usually to blame when horses challenge fences. Virtually any horse can clear a five-foot fence from a standstill if he's sufficiently motivated, but very few will be keen to clear two which are only eight to twelve feet apart. This also virtually eliminates squabbling, or courting, over the fenceline.