2- slow down. Slower speeds allow the animal to get a better look at you. If you've announced your presence with a little shout or talked to the rider, the animal knows you're some kind of person, which should hopefully make them less alarmed about this bizarre thing that seems to run without legs. I know that if you're out training, slowing down isn't what you want to do, but it is the safest thing for everybody (this goes for if you are driving a car, as well- pass wide, and slowly. do not honk the horn, LOL)
3- If the horse starts jumping around, ask the rider how you should proceed. They may simply need a second to get out of the way and stopped so you can pass safely.