Donkeys and mules are good protection for a herd. They show no fear of anything and will attack even large cats to protect their herd. I know several people that have them in with their minis for protection.
Please read the last Brayer. It had a good article about Donkey's protecting.
Want to also let you know,, at the old Farm in Kitsap County over at the bottom of Pine rd.
There were these folks that ran Scottish Black faced sheep and had a standard donkey to guard. Was a good guard donkey, protected the sheep from dogs. However, it was killed by the Cougar there. Cougar also killed and ate two of their sheep afterwards.
No prey animal is going to be able to protect anything from a Cougar. Donkeys and Mules are prey animals.
They can be hurt or killed just like any other.
I do not mean to be a wet blanket,,but having friends that have had Donkeys killed in the line of duty...
Many of them no longer use equines to protect there flocks or herds of what ever.
First,, the best protection is good fencing.
If you are lucky enough to have a solid barn also a plus.
Learning how to shoot and kill in a proper manner any animal that is out to kill your livestock.
Then a LGD....but some bring their own problems.
Donkey or mule as a guard would be last on my list. Standard or Mammoth might be able to take on 1 or 2 dogs or Coyotes.
But in no way could they take on the packs of aggressive Coyotes or the cougars we have here.
I have a mini Donkey. I do not expect her to do any guard work. She does what I want,, she will bray when strangers come around. For any thing else she gets scared and stands there and has the runs.
I will take care of the predators.