Found tottering alone in the desert with their ribs visible and their heads hung low, horses play a backbreaking, unappreciated role in the multibillion-dollar drug smuggling industry created by drug prohibition. Mexican traffickers strap heavy bales of marijuana or other illegal drugs to the horses’ backs and march them north through mountain passes and across rough desert terrain. With little food and water, some collapse under their heavy loads. Others are turned loose when the contraband gets far enough into Arizona to be loaded into vehicles with more horsepower.