You can also breed them (though you can't breed mules without horses, etc). There is a cycle in this equation where you can breed replenishment donkeys once from a subset of them every year or so, and they go into productive use after about 2-3 years. Depending on the length of your campaign, you can extend your range by eating older donkeys as jerky and soup and breeding new ones. Setting out with a bunch of pregnant donkeys could also extend your range and campaign. Add goats (who can even ride donkeys) and you have a renewable source of protein via milk and cheeses.
Horse gestation is marginally shorter, but you can also milk them to feed troops, as mogolians did. I don't know if you can milk donkeys, and mules less likely given their sterility.
This is complex enough that it could become a game.
This is complex enough that it could become a game.