It would be good to have more input. Some horses spook easier than others. Some riders take their horses out on busy roads so the horses will get used to various scenarios.
There are horses on Overall. They become skittish regardless of our speed, regardless of whether they are on the road or in the pasture - even when they see us. We slow down (location is also on a hill so slow is natural) & the riders work to control the horses.
My wife who is a horse person said the best thing for bicyclist to do is let the horse rider know that you are there about 4-5 car lengths ahead and move to the other side of the lane.
Bill