This is an unusual area of etiquette we don’t normally discuss but being that I am a horseowner, I find myself wishing non-horse people knew better etiquette around animals large enough to inflict serious injuries.
The following video shows a rather epic wreck in a show ring with loose horses dragging harness and carts behind them. The first horse starts the problem when he gets stung by hornets near the far end of the arena.
Every time a horse collides with people or other horses, the audience screams. For a horse, this has the same effect as pouring gasoline on a fire. They hear it and assume there is danger they must flee from. If you are ever in an arena or at a parade route where there is a loose horse, BE QUIET! (With the caveat being that screams to warn someone in imminent danger of being trampled being OK.) Don’t flail your arms like a windmill chasing after it either. I’ve seen a lot of you tube videos where people think they are helping but what they are doing is driving the horse forward to continue running. Stay quiet, get out of the way and let the professionals handle it.
The announcer in the previous video doesn’t help the problem with her emotion-laden voice and conflicting directions.
Contrast the former video with this next one. The announcer calmly and immediately commands the audience to be quiet repeatedly and they comply. Several tons of horseflesh gets under control rather quickly because the audience doesn’t get out of control. There are a dozen horses or more wearing blinkers which prevents them seeing behind themselves so if the audience behind them had erupted into screaming, they would have panicked wondering what horrible thing was coming from the rear to get them.