As a horse owner as well as a cyclist, I've found that both my horses never hear bikes coming up behind them when out on a ride..... one minute we'll be wandering on, the next minute they've swung round as they've been suprised to suddenly hear the 'wistling' sound coming up behind them......since horses natural instinct is to run from danger, that's generally what they do ! the art is to stay with them when they swing round or gallop off.....

I think that both horses and cycles should be able to manage to share tracks and trails etc... From my personal point of view i'd rather a cyclist make themselves aware to me before they are right behind me & pass me with caution, so that I can get prepared to at least show my horse what's coming up behind her and to get out of the way so the cyclist can get passed safely without the potential of being squashed by my horse................. In return for being made aware that a cyclist is coming , i'd always get of their track /road to allow them to pass and to allow myself space if my horse decides to take off.......

Just my humble opinion........

although I know plenty of horse riders & cyclist who think that niether should be on their track !