A pack donkey is a donkey used to carry gear, suitcases, or other items people don't want to carry themselves. This link should give a picture of one. :) http://www.trekearth.com/gallery/Africa/Morocco/North/Tetouan/Chefchaouen/photo202896.htm

I would guess if your being referred to as a pack donkey it could be a way of your carrying everything. A literal "pack-Donkey" would be a donkey used to carry gear.