These beasts stampede across the vast planes of hell, ignorant to the Great Conflict they are often enlisted to fight for. For the assault on Mt. Arreat, Baal had his personal metal workers construct plated armor with riding harness on the beasts. Often times guided by Imps, these beasts are almost impossible to control. It's thought that the Imps don't actually control the Beast, merely use their height to get a better vantage point to fire from. Either way it doesn't appear that the Siege Beast is aware of the passenger.