The Romans used the Molossian breed of dog to guard their property that were trained to sleep indoors during the day and keep watch at night. The sign Cave canem was necessary as these dogs werer trained to kill. The molossus dogs were used in the arena to fight lions, tigers and even elephants and was used by the army as a fighting dog against their enemies. It is no wonder that the sign "Cave canem" was used so often in the private homes of Romans!
Signs stating 'Beware of the dog' rather than 'Cave canem' are used in properties that have dangerous guard dogs such as Dobermanns, Rottweilers, Wolfhounds, Mastiffs, Bull Terriers and German Shepherds.