A messenger dog with a spool attached to a harness for laying out new electric line in September of 1917. From Animals at War; World War I in Photos.
[A]utomobiles, tractors, and trucks were relatively new inventions and somewhat rare. British and French forces imported horses from colonies and allies around the world, a near-constant flow of hundreds of thousands of animals across the oceans, headed for war. One estimate places the number of horses killed during the four years of warfare at nearly 8 million. Other animals proved their usefulness as well: Dogs became messengers, sentries, rescuers, and small beasts of burden. Pigeons acted as messenger carriers, and even (experimentally) as aerial reconnaissance platforms. Mules and camels were drafted into use in various war theatres, and many soldiers brought along mascots to help boost morale.