I hate it when I biking down the road and suddenly another cyclist just zooms past me without so much as a warning. I don't always hear them, and they often don't give much clearance for if I was to suddenly swerve for some reason. It also startles me sometimes. One time a guy on a bike did this when I was in the middle of a busy intersection, going straight. He almost gave me a heart attack because at first I though he was a car coming up around me to cut me off. As he flew on past me I only had time to shout out "You SCARED me!" Like he cared.
I ALWAYS call out something when coming up behind someone who might not hear me- whether a jogger, another biker, whatever. I think it's common courtesy and common safety as well. A horse is sometimes unpredictable, just like a small child in the road- you just don't come flying past them with no warning, it would be inconsiderate AND dangerous. I call out a cheery Hello way ahead of time when I see horses up ahead- best to let riders and horses both know well in advance if I am coming up behind them.
When we have a choice, why not do the safer thing???