Thank you for that poster! Heartrending.
It has been stated that eight million horses died in WWI.
Remember that, at that time, the internal combustion engine was still in its infancy, and most heavy work was done by horses. It was the last major war that employed horses in quantity. My dad was in the last battalion of the 2nd Cavalry Division to actually ride horses . . . and they left them behind when they were shipped overseas in WWII.
Most of our horses now are high-end show, race, and rodeo horses, or basically family pets -- no more work horses that were simply cogs in a machine.
They were the equivalent of the wrecked tanks and jeeps and trucks that you see by the side of the road in almost any war since.
Most people just treated them as expendable equipment rather than living creatures. The English and the Americans were almost alone in having warm feelings for their horses and dogs. The French even remarked on it (that painting is based on a French account)