Horses, mules and donkeys were used during WWI to transport ammunition and supplies to the front. They did so during the horrors of shellfire, through terrible weather and appalling conditions.
Eight million horses, mules and donkeys died during the war. In this photograph, taken in 1918, American soldiers pay tribute to the fallen animals.
Source: http://www.animalsinwar.org.uk/index.cfm?asset_id=1375, https://www.reddit.com/r/OldSchoolCool/comments/6br5s3/american_soldiers_paying_tribute_to_the_over_8/