That scene was actually "real" in that they actually made the horses crash through the glass. I remember that they were very proud of that stunt and talked a lot about the difficulty in shooting this scene. It was mentioned that they had to get it right the first time, since the horses would learn from the experience - so to speak - and would not be willing to do it again!
Today, I am sure the anti-cruelty to animal guys would be after them, and instead there would be CGI riders on CGI horses crashing through CGI glass.
I'm sure the window glass was spun "candy-glass" and the frames were light balsa or bass-wood. I would have been fairly difficult for the horses to be hurt much.
Clint Eastwood led the fight to establish a code that prevents injuring animals to make a movie.
Apparently that code was not in place during the making of this movie. Not only do the horses crash through the glass, but when they fall, they fall forward, on their heads. This means they are being tripped, probably by a wire.
When the horses fall sideways in a movie, they are not being injured because this is a normal movement for them. When they fall forward, however, there is a strong risk of injury or death.
With CGI and animatronics, mistreating animals is not necessary.
We report, you decide; but we decide what to report.