What is the max weight a miniature donkey can carry?

Any equine, including donkeys and mules, can pull their own weight (cart n harness animals) and carry 20% of their own body weight. So if your mini is 300 lbs he can 60lbs (that includes tack as well). Donkeys are reputed to being a bit stronger than horses, but the standard rule of thumb is they can pull how much they weight and they can ride 20% of what they weigh.