So, I was having a conversation with a friend and the subject of horses in movies came up.
I told him only female horses were featured because they didn't want to have the schlongs on screen. I stand by this. Can you give me an example of a male horse in films?
That is part of the concern of using males, but my understanding is the main reason they use mares is because stallions are too aggressive. If there are other stallions they tend to fight, but even in a group of females a stallion will be aggressive, asserting dominance and making sure the "herd" stays in line.