As for interface and parameters, I don't see what might be missing, as the donkey would only stand, walk, run and die. He does nothing which human mercs don't do, so it doesn't need any special user interface. You could even live with the standard human merc inventory, since I guess that part would need some re-coding: Just give it an (irremovable) "donkey pack" backpack. It would look a little silly displaying the wrong body shape, but that's just cosmetic and wouldn't affect its use.