Coughing loudly behind an unsuspecting horse can also be quite effective. Most horses do not know what to think when you stand on your hind legs for more than 5 seconds at a time. Most horses also do not know what to think when you climb into the salt feeder. Or the grain feeder. Neither do most people.
Any open object is meant to be climbed into.
Never mind the size of the object in comparison to your size. Also never mind that the object may be harder to exit than it was to enter. Trailers are more fun to get into than they are to get out of. Horse hair bears a striking resemblence to grass. So does people hair.