When Otis hops over the rock he tripped on before his talk with Ben he says "See? I learned!". This ties in with a theme of the film. Some lessons are harder to learn than others. And if you refuse to learn one lesson it's just going to be harder on you.
Ben says that Otis will never be happy if he just spends his time goofing off. Otis is only completely happy after the death of his father when he accepts responsibility of being farm leader. Even more Fridge Brilliance sets in during the very final scene. Otis looks up at the sky to see the stars dance after he's assumed responsibility of leader and father just like Ben did when he was young.
It's really not that surprising that Miles was so good at fighting the coyotes who had the bright idea of trying to gang up on him. Donkeys and mules are often used on real farms as livestock guardians, since they are shockingly good at detecting and killing predators, including coyotes. It's very possible that this protective rule was even Miles' previous job on the farm before he became old enough to retire, which could have been one of the reasons he seemed to be so close with Ben.
