In real life, one arrow (no masterworkd) kills a horse, even if he fall back.
Lol wat
Have you ever been near a horse in real life, they are fucking tanks. One arrow isn't going to stop one of those, especially the big arse muscle beasts they bred for war in the middle ages. It's part of their nature to run harder and faster when they are injured, the only thing that will probably stop them is an arrow between the eyes, a really big pointy stick which is braced or a broken leg, even then they still try and run.
Don't talk out your arse if you know nothing about horses.