Let's try "Pack Horse" on for size!
I get you and nice try, however no. But I would have to say Pack Horse would not do. Now forgive me and feel free to correct because my equestrianism is dated and far more English than Western, but let me set a scenario for you:
You and I, Vern, are about to set out on a big trail ride. Maybe even in a posse hunting the them other mules. However, I miscounted and we don't have a saddle horse for you. I suggest you ride a pack horse. I think your reaction might be, "hey, why do I get the crappy pack horse instead the high quality saddle horse?"
Hence Pack Horse refers to a horse not living up to its potential. I do not wish to be called a pack horse.
On the other hand, if we were handing out mules for the trip and someone said, "Vern you want this mule? It carried 1000 pounds of gear up a sheer rock wall last week", I think you would said, "Why heck yes Pard, that must be the BEST mule around these here parts. I imagine he can carry, my mixer, lime tree and portable tequila distillery. Everything I need for a long trail ride"
(Perhaps you might not have said that last bit, but you get the idea...)
When I was hauling 2 50 pounds bags of medical supplies through the airport and then down to the ferry, I was that prize mule and proud of it!
And don't walk behind me.....