Etiquette when passing horses

Last time I rode a horse on the road lorry drivers would signal their displeasure at seeing me/the horse by blaring their horns as they sped past without giving me any extra room. My horse was fine, therefore the horse (and rider) Fox came across needs to HTFU.

This, a bit. doesn't sound like the OP did anything wrong. Some people like to be warned, sometimes any warning you shout will come accross as bossy. I slow down to about 10mph or less, and say 'I'm going to pass on your right', which means they know exactly where to expect me.