That horse really took a tumble, didn't it?

I just saw the movie. Can't really answer your question. I do know the scene was filmed at a different speed, with the action really sped up when viewed. The horse was not nearly going so fast when it fell.

Sadly, animals were treated like disposable props in movies back in those days. It's hard to watch at times. Horses, in particular, really got a raw deal with all the old Westerns.