It's unfair, and downright discriminatory, that other animal balls are offered in the pro shop
After reading your post, I did some poking around. It appears that the Donkey Ball has not been an option as it would then necessitate the need for an Elephant Ball and may lead to political unrest.
Furthermore, to appease the ASPCA, no animal balls of any kind will be offered to members to play with. It is not only cruel to the animal but just wrong.
However, do not despair, Biodegradable, eco-friendly balls will soon be available.
EDIT: Google it, this crap is for real!!! They even have one that dissolves over time.