What happened to the animals in WW1?

Answer 1

Many animals such as horses, ponies, and donkeys were used for transport of military equipment, food, and so on.

Horses were used by Mounted Infantry and Cavalry but, as the Western Front was usually static, they were not often used.

Dogs were also used for various purposes.

Answer 2

After serving bravely in the war, most of the horses were sold in Europe probably quite inexpensively. Shipping most of those horses all the way back to the States would be expensive. There were also mules and dogs. With few exceptions once the war was over the value of these animals greatly diminished.