Here in the southern US, donkeys have been being used as guard animals in livestock herds for a number of years. They particularly don't like coyotes and rattlesnakes.

However, local wisdom (aka conversations overheard at the feedstore) is that rescue animals, especially abandoned family pets, don't adapt well to their guardian role. They need to be introduced to the livestock herd at a young enough age to bond with it. Otherwise, they tend to be loners and may even fight with the animals they're supposed to be protecting.