I have two donkeys who live with the horses. Not many predators in my current area, although the donkeys chase anything that comes into the field- large birds, cats, dogs, deer, etc. They’ve never done any harm to anything, to my knowledge. Not sure how truly effective they are in terms of protection, but they do tend to bray if they feel something is amiss.
All of my horses love the donkeys and consider them part of the herd, although it took many years for my one mare to warm up (despite living with donkeys on/off since birth). I originally got my first donkey as a companion for two flighty horses that moved home and she calmed them down considerably.
They get treated more or less like my horses in terms of maintenance: same vaccinations, same trimming schedule, same deworming protocols. They don’t eat much: hay, a handful of something to carry supplemental vit/mins. They tend to be more sensible than the horses and silly injuries have been rare (but not nonexistent). They don’t “do” weather; rain, wind, or anything extreme, you’ll find them in their shelter. I blanket and sheet them as needed. In the summer, fly boots have become a must or else the flies eat their legs alive. I’ve also needed grazing muzzles in grass season.
Training-wise, they are quite different from horses. If you think of horses like dogs, donkeys are more like cats. Not that they aren’t trainable, but their default temperament is to please themselves, not an alpha. You have to make it worth their while. They also don’t appreciate repetition.
I adore my donkeys! They make me smile every day, even the old, curmudgeonly fellow. I can’t see myself ever being without donkeys.