Initially I read this as the GM was being a donkey...
I don't see any problem with using a pack mule or a donkey. There are realistic problems that arise from them (terrain, taking them inside etc.) but there are also advantages (treasure holder!). There is a reason why donkeys were (and still are) used to carry stuff, don't worry about it too much, it's just part of the game. If the situation calls for the party to be unable to carry large amounts of equipment, put it in a location where it's inaccessible to the donkey or just kill it. As the GM you are GOD of the world, something as small as this doesn't need to be a huge issue.