I own, and have owned several horses thru -out my life.

I ask bikers, or even people walking to say hello, say the horses name, and then the horse can recognize you as a non threatening human- not something to fear.
Yes, horses are unable to see directly in front, or behind them, and can be easily startled.

Many of my horses have had plenty of experiences and don't even glance at cyclists, deer, ATV's, etc... However, the best policy is to slow down, and ask the rider what the horse needs you to do in order to keep it from spooking, and causing injury- just common courtesy