Thanks for the input. I asked a couple of horse guys I work with and they both said that I probably did the right thing. They also said that no one should ride a horse on a public road/trail that is skittish when confronted by vehicles. Still, I feel as if I should have at least shut my bike off and asked about how they would want me to pass, which is what I will do the next time.
I'm personally not a horse person having had some bad experiences when I was a young. I decided early on that dirt bikes were a lot more safe given that if I crash, it would typically be my fault and not due to an animal many times my size. Still, I respect someone's right to ride and here in Arkansas, horse riders are for the most part, pretty nice people. We have a lot of multi-use trails in our National Forests and the mountain bikers, dirt bikers and horseback riders seem to all do their part in keeping the trails open (maintenance, etc.).
As far as damage caused by horses, their impact is far less than the redneck ATVers I've seen.