First, you don't want a horse crashing into your fence, so he must be able to see it. Second, if a horse does hit the fence, you want it constructed so that he does not get hurt. The company Premier 1 suggests asking the following questions on choosing the correct electric fence system. Most questions can be adapted to ask about any fence type. Choose a design that will repel the most difficult.
Stallions are always harder to fence than mares or geldings.
This is most true during breeding season when the sex drive distorts normal pain response reactions. Buy and build for the worst-case situations. Young animals and their mothers are desperate to rejoin during weaning. Bored animals in corrals will challenge fences more than those on pasture. Finally, nothing is foolproof.