Moth balls will repel snakes. For foxes I've heard of people sweating in a shirt so the material stinks of BO and unwashed human hair being tied around the coop to repel foxes. Their smell is a priority sense so they get scared there might be a human in the chook house! I've heard cat's and know dog's HATE white pepper if you dust it around outside the coop... and thats all the track making predators we get down under so I can't help you any more!
You could try a movement sensor light over the path near the coop so the mystery animal thinks it's been sprung and a human is about to attack!