They also served

 Horses were used extensively in the First World War. Horses were also used in the Second World War, mainly by the German and Soviet armies on the Eastern Front. They were employed in the millions and died in the hundreds of thousand. They gave their all when asked. They had no choice and did not know better. One could say the same about the men who fought and died.  There are many memorials to the horses who fought in many wars and so there should be. They were as brave as the men and contributed as much.

Wikipedia (https://en.wikipedia.org/wiki/Horses_in_World_War_I) says that:

One estimate puts the number of horses that served in World War I at around six million, with a large percentage of them dying due to war-related causes.

The military used horses mainly for logistical support; they were better than mechanized vehicles at traveling through deep mud and over rough terrain. Horses were used for reconnaissance and for carrying messengers as well as for pulling artillery, ambulances, and supply wagons. The presence of horses often increased morale among the soldiers at the front, but the animals contributed to disease and poor sanitation in camps, caused by their manure and carcasses. The value of horses and the increasing difficulty of replacing them were such that by 1917, some troops were told that the loss of a horse was of greater tactical concern than the loss of a human soldier. Ultimately, the blockade of Germany prevented the Central Powers from importing horses to replace those lost, which contributed to Germany's defeat. By the end of the war, even the well-supplied US Army was short of horses.

Conditions were severe for horses at the front; they were killed by artillery fire, suffered from skin disorders, and were injured by poison gas. Hundreds of thousands of horses died, and many more were treated at veterinary hospitals and sent back to the front. Procuring fodder was a major issue, and Germany lost many horses to starvation... Battle losses of horses were approximately 25 percent of all war-related equine deaths between 1914 and 1916. Disease and exhaustion accounted for the remainder

 Wikipedia (https://en.wikipedia.org/wiki/Horses_in_World_War_II) says: 

The role of horses for each nation depended on its military strategy and state of economy and was most pronounced in the German and Soviet Armies. Over the course of the war, both Germany and the Soviet Union employed more than six million horses.

Horse-drawn transportation was most important for Germany, as it was relatively lacking in natural oil resources. Infantry and horse-drawn artillery formed the bulk of the German Army throughout the war; only one fifth of the Army belonged to mobile panzer and mechanized divisions. Each German infantry division employed thousands of horses and thousands of men taking care of them. Despite losses of horses to enemy action, exposure and disease, Germany maintained a steady supply of work and saddle horses until 1945. Cavalry in the German Army and the Waffen-SS gradually increased in size, peaking at six cavalry divisions in February 1945.

The Red Army was substantially motorized from 1939 to 1941 but lost most of its war equipment in Operation Barbarossa...The logistical role of horses in the Red Army was not as high as it was in the German Army because of Soviet domestic oil reserves and US truck supplies. 

German and Soviet armies relied heavily on work horses to pull artillery and supplies. Horses seemed to be a cheap and reliable transport especially in the spring and fall mud of the Eastern Front but the associated costs of daily feeding, grooming and handling horses were staggering. In theory horse units could feed off the country, but grazing on grass alone rendered horses unfit for work and the troops had no time to spend searching the villages for fodder. Hard-working horses required up to twelve pounds of grain daily; fodder carried by the troops made up a major portion of their supply trains.

Horses needed attendants: hitching a six-horse field artillery team, for example, required six men working for at least an hour. Horse health deteriorated after only ten days of even moderate load, requiring frequent refits; recuperation took months and the replacement horses, in turn, needed time to get along with their teammates and handlers. Good stables around the front line were scarce, makeshift lodgings caused premature wear and disease. Refit of front-line horse units consumed eight to ten days, slowing down operations.