Donkeys are good, but as far as upkeep they're as high-maintenance as a horse, even more so since they will founder more easily than a pony on fresh pasture.
Lamas are nice too, and you can get wool off of them (Which people will actually buy raw if you don't want to do anything with it). If you can find one affordable, alpacas are even better. But both will sometimes bully the other animals, especially foals--I know mine do. They are lower maintenance than a donkey though.
You may also consider goats. Watch out for breed though, Boer and Nigerians will be very aggressive, even the wethers (neutered males) and females--trust me, I have had both breeds. The ones I like are Angoras, which are wooly and they have really gorgeous, spiraling horns. The breed standard for them is to keep the horns on, so you won't likely find any dehorned (not that they'd be much good protecting without them anyway). They are mean to dogs but kind to people, I can even lead my Angora billy around by his horns, which have over a three-foot span.
When we had a coyote problem, we tried all of the above, and even geese (for our chickens) and had found that all of them protect just fine, it's only the matters of maintenance and socialization that need to be factored.
I hope you find what you're looking for!