In reply to MG:
So nice to see some horse-rider bashing going on. In other news, all climbers are mindless pricks who ruin the outdoors for everybody else by covering it in chalk, and wearing ludicrous head-gear.
Anyway...
You'll find those that glare at you are probably in the 'happy hacker' brigade - i.e. enjoy their horse for going out and about, are scared if the horse dares look at anything (god forbid it has a brain), and spend most of their time talking about what their horse could do, rather than actually learning how to train properly.
They piss me off, especially when they ride two-abreast despite knowing other traffic is there. They are, on the whole, the ones who will glare no matter how considerately you pass. I've been known to stop my car and suggest to these kind of riders that they might want to acknowledge those who have passed considerately, as it makes like rather easier for us all in the long run.
Then there are the considerate ones. I know horses can do funny things, I don't get angry when they do, and give people a cheery wave (or a nice 'hello' if it's a bike rider, for example). The best thing on a bike is to shout a bit behind 'bike coming', or words to that effect, and then pass slowly. On a car, just give it a couple of metres and pass carefully.
Of course, there are those other road users who steam past at some silly speed, liable to wipe out horses, pedestrians and bike riders alike. They are just cocks.