Good point, Widowmaker. And like men, the horses and mules had to suffer terribly in that brutal war. Perhaps even more so, because they had no idea what was going on. (But that must have been true with many men also - it can't have been all that clear all the time just what the hell the generals and politicians who started the whole mess were thinking and expecting from their subordinates!)
Horses were extremely important for the majority of armies even in WW2. Only the British and American armies were completely motorized, the others kept using horses in addition to trucks throughout the war. Even the Germans and Soviets, who we always see in propaganda films charging forward with almost nothing but tanks and APCs. In reality, most of their troops marched on foot just like their ancestors in countless earlier wars.