I don't know if mules are like other equines but if so, then presumeably you'd need two

Plus, some ponies are pretty aggressive towards sheep, and I certainly wouldn't want teeny lambs in a field with those great hooved feet

People do use llamas and alpacas as guards against foxes and some do say they are very effective. I understand that they can be aggressive towards people too, which may be a good thing if your land is completely private, without rights of way etc. One would assume they'd chase dogs off too, but maybe best to go and have a trawl through the threads in the camelids board - I'm sure this will have been discussed before.
If we had larger areas, ring-fenced, I'd have been experimenting with Livestock Guardian dogs before now - Maremmas and the like. But where I am it's not feasible; it's relatively small fields, other people's ground all around (and in one or two cases, in between), public footpaths and two roads running through. It might work for you though?