Yeah, guard donkeys are used quite often for both sheep and goats (and I guess cattle, too) in Texas (and probably elsewhere). I would recommend getting one that is used to and gentle with people--and likewise hell on varmints and unwanted critters!
As I said, my parents had a guard donkey left on their property after some goats were moved off (none of the animals were theirs). Sally (the donkey) would follow my mom around, and one day chased off . . . . something . . . . that my mom never even caught a glimpse of.
They're interesting animals, but some do have a stubborn streak and/or can be mean to people. So be choosy about which . . . ahem . . . donkey you bring home! (It wouldn't let me use the other term!)