how??
Hooves and teeth and sheer determination. There is a video floating around of a donkey swinging a mountain lion around by it's tail.
Teeth and hooves will easily kill anything 200 lbs or less that walks the earth.
Controlling the coyote population - BBC wildlife (video 1:46, short video of donkeys and llamas protecting sheep from coyotes)
Donkey kicking camels (video 0:14)
Horse Versus Donkey
(video 5:57 - 350 lb donkey giving 1,250 lb horse pure hell. Neither one gets serious though they're just playing.)
The same way one just killed the mayor. They’re pretty territorial and don’t tend to run away from trouble they can kick into submission instead.