There was an (in)famous report in the UK a couple of years ago where some do-gooders tried to stop microlights from being based out of a strip near a field that regularly had horses in it. Main thing was they did not go talk to the stable owner before starting their campaign. When the reporters spoke to the stable owner his comment was that he has loads of fields well away from the microlights and that he deliberately rotates his younger horses through the fields near the airfield before anyone rides then on the road to get them used to vehicles whilst they are on a large open space, before they encounter vehicles in the comparatively confined roadside locations. The do-gooders kind of went quiet after that
Oh back on topic: There is a biker cafe near me that is also in a horse riding area so you at times we can talk to the riders, most of them said that cyclists (particularly road bikes, but sometimes mountain bikes) are usually more of a problem because they are quiet, fast moving and surprise the horse. All they wanted from motorised vehicles was a slow speed, low revs (including knowing how to mimimize any exhaust popping) and to give as much clearance as is safe to do so.
To me as much clearance as is safe on some narrow roads is stop and wait although that does assume that the horse is coming towards you.