You know what? Stuff happens. People get unlucky.
Having a saber cat chase your horse away is something I could imagine happening if Skyrim was 'real life'. It seems like a very possible situation.
Frustrating? Perhaps. But realistic.
Why do people have to even make a point about realism? It's a game and there's no reason why we can't have the ability to either call our call to us, or at least see it on the map. Fair enough a Sabre cat would scare a horse away, but this game isn't entirely realistic anyway, so I'd rather see my horse do a Hogan Leg drop on Sabre Cat, then we both call it a day and ride off into the sunset.