Horses
Horses may have seen obsolete with the introduction of cars and vehicles but during World War II there was still a desperate need for horses for the war effort. The Wehrmacht horses were vital to the German war effort because the Germans were lacking in oil resources for all their military vehicles. They needed the horses to pull their equipment and help transport troops.
For most of the allied forces the military was mechanized by 1928. However, the United States had a mounted unit in the Philippines and the German army maintained a single mounted brigade. The French incorporated mounted troops into their mobile units and the Soviets had thirteen cavalry divisions. The Italians, Japanese, Romanians and Poles kept substantial cavalry divisions as well.
But during World War II the use of horses by the Germans and the Soviets increased drastically. As many as six million horses were necessary to keep the Red Army and the Nazi Army moving. The Soviets had mechanized by World War II but much of their equipment was lost in Operation Barbarossa which meant that they were forced to go back to using horses to move artillery, troops and equipment. For the Soviets, they soon ran out of horses as well but with the help of the Allies and increased tank production they were able to keep mobilizing their army.
The Germans on the other hand relied on horses for the entire duration of the war. They kept hundreds of thousands of horses on the battlefield and kept up supply until 1945. There were substantial losses of horses for the Germans with a loss of 179,000 horses just between the months of December 1941 to January 1942. After World War II the use of horses disappeared almost entirely from all of the developed militaries. Today horses are largely just used in a ceremonial sense by major militaries.