Not that some dogs and horses don't get along, but not all do, and even those who appear to be friends need to be able to get away from each other. If confined together without supervision, all kinds of bad stuff could result - horses bitten or dogs kicked and stomped. Horses basically regard dogs as predators. They can learn to accept a dog into their "herd" but it is not always an easy relationship. I think they need supervision and/or room to get out of each other's way.