Is it illegal to honk or rev your engine when passing a horse on the road?

It would be in some states.

In some places yes.

It would be, and moronic too.

pretty stupid and irresponsible........

prolly

i think so !, mostly its a law in the Amish cities, where horse and carts are still legal

In the US there are 50 states, each with their own motor vehicle code. Not to mention the numerous countries in the world. You need to narrow it down a bit.

Yes, and pretty stupid, if the horse bolts and the rider or horse gets injured you will be responsible for any medical bills.

No but it's a dick move