Hi Roger, a standard size donkey (mini's are 36" and under) would be able to keep foxes, etc., away....instinctively, donkeys do not like canines of any kind, but there are some, I suppose, who tolerate those that they 'know'. I think a real guard animal will take its job seriously.
Perhaps the reason people end up putting equines in with cattle is lack of separate pastures for the equines...at least, that's why we've had to do it...but I've seen our smallest jack (30") back on up to the bull eating grain from a bucket, kick a couple of times, and the bull leaves the bucket (and the donkey gets the feed he's not supposed to have). We don't use medicated cattle feed, but some medicated cattle feeds are toxic to equines. I don't want our donkeys eating the cows' grain anyway...some donkeys get fat on air. So I don't like having donkeys and cows together for lots of reasons.
I just hate to see donkeys get a bad rap for being donkeys and doing what they do. I know that some do get along with lots of other livestock (goats, sheep, poultry, etc.). When I let our chickens out to 'graze' in the afternoons, I have to shut the gate to their pen or any donkey who's in the yard will go in and clean up all the scratch feed on the ground. Creates a lot of extra "uh oh's" for me, but I love my donkeys....
Had a bantam hen who hatched out her babies and nested with them in the corner of a donkey stall for several weeks, until they were big enough to fly to the rafters...the donkey never bothered her or her chicks. Another hen walked through our calf weaning pen with her babies and one of the calves stepped on and squashed a chick...not intentionally, just never noticed them!
Now, I like our cows too (most of the time), but every species is different. Have learned so much about animals and nature (good and bad) on this farm....get ticked off about a lot of it, but wouldn't trade the experience for anything.