Mules run on partly celulosic biofuels, which they convert directly into mechanical energy at the point it's needed. They include advanced elastic shock-absorbers which actually return energy for the power stroke. They have autonomous capabilities and vision systems that put any robot to shame.
Robotics is trying to imitate all of these aspects, and is probably making great strides. But if I want to carry something over a mountain pass, give me today's mule over the 8-years-from-now robotic mule any day. Wheels, propellors, jet engines, are a way to beat nature, because evolution isn't very good at those things. But four-legged travel has been optimized by nature (and slightly reoptimized by human breeding to carry burdens). You won't beat it with any foreseeable technology, and you won't make the unforeseeable come any faster with research in this area.