What's with the galloping in the yard?

Hi friends, I have always wondered what the galloping in the prison yard is, is there even a canon answer? I've spend a good bit just running around trying to find whatever is the source of the noise and I cannot find it, I assume you simply cannot no matter how hard you may try. But I still must wonder, is it a horse? Somebody on a horse, is it some weird kind of enemy. If anyone knows I would appreciate some sort of input :)