I would have to go with a human on this one. Think about it, if you use a horse, you have to feed the horse and the human used to get the horse to do something and you have to pay the human to work with the horse. The human you get to drive the horse needs some knowledge on how to properly get the horse to do what you want it to do which means you can't hire unskilled labor. You also have to build and or pay for housing for the horse... pay for grooming... pay for medical care... the list goes on.
If you could get a human to do the same thing, just hire an unskilled worker, cast your mojo on him/her and hand him/her a quadruple size bowl of cheap porridge. The work will be done better because a human has more precision doing things manually than they do driving an animal to do things, and you eliminate 100% of the many costs associated with the horse.
Just in case it wasn't obvious, the energy that I am proposing the human use to get around the conservation of energy bit is the calories in the extra large bowl of porridge. So the human would get the requisite energy much the same way as the horse would, by consuming food. In fact, if you wanted to go really cheap you could always cast a ruminant stomach spell on the human and feed him/her hay... but you might end up with some disgruntled employees.