I live right on the edge of the city, about 1/4 mile from a hacking stable and believe me, they ride on the pavement, some even being encouraged to do so by the leader holding the lead rein. The very fact that someone's horse is being led seems to indicate that they are less than competent. If the problem is with the horse, then that horse shouldn't be allowed on the road - maybe there's a case for there to be a test for equine competence?
100 yards away (towards the stables) is a proper, well maintained bridleway that runs for about 1/4 of a mile to the bottom of a little valley - it's a lovely route and I've cycled it many times to avoid the road (never met a horse on it); 100 yards the other side is a FOOTpath that ends up in the same place with a barrier to stop cyclists 1/2 way down it - walking down that FOOTpath, I've met plenty of horses - some of them (and riders) several times.
Driving down the road a couple of years ago in my LandRover, I saw a group of horses on their way up (on the road) - among them were a couple of small children. Being a considerate chap (despite the comments above, I am really!) I pulled over and stopped the engine to let them pass without the distraction of a rattly engine. I'm just glad it was a Landy since one of the adults' horses got a little skittish and hit the side of the car with the stirrup a couple of times. From the look on the rider's face, it was all my fault...
I'm sure there are some riders out there who DO wear reflective clothing and behave correctly but there are precious few here!