Llamas and donkeys are sometimes used as "guard animals" because they are bigger then the sheep and goats and donkeys especially are very dog aggressive.
Additionally if you get a llama or a donkey you need to make sure that they have been around sheep and goats because not all will be good guard animals.
For donkeys in particular, look for a female preferably one either preganent or with a small foal. Those make the best guard animals because they defend the foal.