Something on another thread reminded me of this.
I know of three different ways that supposedly worked.
When my dad was young and still on the farm, they had a boar that would kill every chicken that got in his pen.
He caught a big old owl with a broken wing and threw it in the pen with the old boar., he just did it, he wasn't thinking it would cure the boar from killing chickens. He claimed that hog was still sqealing at 2 in the morning because that owl had latched on to it's nose with both feet. lol
He claimed it would run to the other side of the pen when a chicken got in it's pen after that.
I heard (didn't see) of a farmer that slit his hogs eyelids, so that they were on the bottom of their eyes, supposedly anytime they looked down at a chicken their nose was on the ground, so they couldn't catch them.
This one I did verify.
My wifes grand dad hung a dead chicken up by a live 110 volt wire in the hog pen for a couple of days. After that when they fed the hogs, all the chickens would jump in and eat their fill, while the hogs stayed up against the other side of the pen, waiting for the last chicken to leave before they would eat.lol