For obvious reasons, you should always tie a horse with a knot that won't get tighter if the horse pulls against it. It is also important to be able to free your horse in a hurry if he is panicking or if an emergency arises. See the following illustrations for two variations of quick-release knots.
Note: When tying a quick-release knot, make sure to keep your fingers out of loops to avoid getting them caught if your horse pulls back suddenly. If your horse learns to pull the free end of the rope (thereby releasing himself), run that end through the loop; this is referred to as a "safety" knot. He won't be able to untie himself, but be aware that you won't really have a quick-release knot any more.
See also Knots