There was constant shipping of horses and mules from the US ( very large horse population).
One of the on going problems for Germany in WW1. The removal or labour (conscripts/reserves for the army) nitrates (armaments production) and horsepower from the Rural economy made it hard to feed everyone and the horses. Remember horses pulled everything from the Rail heads, most horses were not in the cavalry mounts. They hauled guns and equipment.
Lack of Horses and poor state due to restricted diet (lack of feed) of those they hand meant the German offensives in 1918 had bigger logistical problems. Moving guns and supplies through no mans land and the past areas of devastated ground (previous battleground, and areas devastated deliberately by the Germans in the withdrawal early) was difficult. Horses or Mules in large numbers was required. (the Germans also had a shortage of oil meaning they were not going to embrace large scale motorization, which would divert material from other sectors of the war economy which is why they also did not really try to build large numbers of tanks)