I slow for horses for two reasons:
First, it could make a mess of my car
And second, I don't want to hurt the rider (well, sometimes I do if he/she is taking the piss)

But what good is a horse nowadays, and why should it be on the road?

If I want to go on the road on a car or bike, I need a licence, and MOT and Insurance AND I have to buy a tax disc. Without any of those, I'm limited to private property. Horses can go on the road.

My car needs a CAT and god knows what else to keep the emissions clean. Horses can shit everywhere!

My car takes me and my family places, but no-one ever seems to go more than one-up on a horse.

Cars tow trailers (and caravans too, sadly) but I don't see many horses doing that nowadays.

Why can't horsey people just realise that the horse is a big toy, and use it in a field like I would use a trials bike?