We got shouted at the other week by a horse rider. Told us to keep the dog under control.
The dog was in the back of the car as we drove past.
OK, he was barking a bit. But a long time ago we specifically put in a lot of effort to stop the dog bothering horses. Where we live there are a lot of working horses tethered up in open areas (particularly the greens where everyone walks their dogs) and as a puppy he would frantically run around playfully trying to capture any horse’s attention. And whilst not perfect in all areas, he’s now exemplary around horses and livestock. Doesn’t even acknowledge their presence.
But occasionally, if he sees someone in a funny hat, connected to a horse like a mythical creature from another world, and they’re chasing us! He gets a little freaked out, in the same way horses get freaked out by him (or traffic, or bikes, etc).
Sometimes it’s difficult. Animals have their own minds and do their own things. Feel their own emotions, and they’re not always easy to control. They’re unpredictable and you just have to treat them as such. Doesn’t matter how well trained or naturally obedient they are.