Most hikers really don't like this sensation, and some become truly panicked and upset. If you have a bell, ringing it probably won't hurt, but you should also speak, so that the horse knows you are a human, and not a predator. This is especially important when approaching a horse from behind, because horses spook more easily in this situation. Ask the equestrian for instructions on how to pass - it may be necessary for you to stop completely and let the horse pass, depending on the width and condition of the trail. Pass slowly and steadily, but only after the equestrian gives you the go-ahead.
Sudden movements can spook a horse.
Every trail user is another nature lover and potential friend of multi-use. Finally, you may wish to read Bill Boles' humorous perspective .