"Why do you always have a bell on your stirrup?"
I get this question fairly often when riding with new people. Nearly every saddle I own has a small bell tied to the right stirrup and its a habit I have kept since I was a teenager.
My favorite riding instructor, Pam, always had a tiny brass bell on one stirrup. She told me about how it was a bit of superstition to keep the fairies away so they didn't spook your horse. The stirrup bell is pretty much the same idea as a gremlin or guardian bell for motorcyclists and offers a bit of superstitious protection to the rider. I enjoyed her stories so it is a habit I adopted from her.
While fairies and goblins aren't a problem, the bell does serve a useful purpose. It helps warn other riders or wildlife out on a trail that we are nearby. Should I get dumped and my horse take off in the woods somewhere, the bell sound might make it easier to track them down. I also find it handy to keep track of other riders who might be on our horses with us. If we are out riding in a group and a student is lagging behind on a lazy horse, I can hear them behind me without having to turn around and loose focus on my own mount.