I think your dog could use some training. Going after an animal 10X its size and weight is not normal and he shouldn't be going after livestock anyway. We had a german shepherd when I was a kid that used to go after horses, donkeys, tried to bite goats and sheep in the throat, pigs, you name it.. every time we went to the country or were at my uncles or grandfather's farm. A mule kicked her and almost killed her. The dog was out of control and a danger to itself. One guy attempted to shoot it because it was trying to bite one of his sheep that had strayed from a small flock he had.
My dad decided to put it in obedience training not because of that but because the dog killed a neighbor's cat and she threatened to sue. He 'd visit the dog at the training facility and train with it almost every day for about 2-3 weeks. The dog's manners were much improved when we got it back. Very obidient and respectful of everyone and everything and much more pleasureable to have around. Still a little protective of us (the children and especially my sister that was 5-6 at the time, I was about 11) but not over-protective like before when she would growl and snap her jaws at any stranger that came within 15ft of us.
