Whoa, whoah, ho. I admonished a beginner with "Don't stay stop, say whoah. The horse doesn't understand stop."
But why is it that whoa, ho, or some similar sound seems to show up anywhere horses are used, regardless of the language spoken by the handlers? I did some research - whatever word is used, it's some variant of the 'oh' sound.
The reason? I have no idea. I suppose we could say "It's tradition" but it seems a very widespread tradition. So, I suspect, the 'oh' sound is something horses respond to particularly well. Some horses are trained to ho and others to whoah, but they seem to make the switch easily (Although I always use the one they're specifically trained to if I know what it is).
Put this one down for "tradition" for now.