There is an urban legend that Pancho Villa rode his horse up the marble staircase of the Gadsen Hotel in Douglas, Arizona. Maybe the citizens of Prescott were hoping to prevent a repeat of this event?
9.36.150 Carrying animals on outside of vehicle. No person driving a motor vehicle shall transport any animal in the back of the vehicle in a space intended for any load on the vehicle on a street unless the space is enclosed or has side and tail walls to a height of at least 46 inches…
Picture thanks to: fatz8016