Quite a funny thread, as this isn't even a topic of debate in lesser developed countries where horses are still frequently used in all types of settings... not just on a rural Wanderwegen.
I always give way to horses, and pass as "predictably" as possible. Also, I avoid eye contact with the horse - I think I read somewhere that they interprete this as aggression.
There is a fair point other have made about the shit though...hitting a huge pile of the stuff mid corner on the bike has some interesting consequences, the least painful of those is being coated in the stuff. Should we introduce horsey-bags?