To ride a donkey, 19.50 body control is required. A newly created character may have to do a bit of crafting, fighting, archery, or archaeology to raise it. Initially, BC 19.5 is shown but may actually be lower and result in "You are not agile enough to control <donkey>. You need 19.50 in body control." when attempting to ride.
* Donkeys cannot currently be hitched to carts and wagons. This is a known bug and will be fixed as soon as possible (per devs).