Even a miniature donkey can carry as much as a big llama. They're harder to train, though. I've never messed with them myself but they can do a good job. Minis are much easier to come by. 2 of them will carry about as much as 1 standard. You're better off having 2 anyway. A lone one will keep you awake all night bellowing for company.
Don't get a jack that's been allowed to breed. The dang things can kick up a real rodeo if they get near a jenny even after you get them castrated. Last spring, we were going through Craigs List to see if any packers were available. Most of those for sale were jacks. No thanks.
