A donkey is used by Cremia to pull her milk wagon. If Link helps Romani repel Them on The First Night, he can help Cremia deliver her milk the following morning, protecting the cart and donkey from the Gorman Brothers who seek to destroy her cargo.
According to the Hyrule Compendium, donkeys are raised as livestock in the countryside. As a result they do not exist in the wild. Donkeys can be found in stables throughout Hyrule, they also frequently accompany merchants who favor them over horses due to their greater strength.