My dog did this to a now ex-girlfriend one time. I had told her more than once to hold the leash by the handle so she could let go if she had to but she liked to slip the loop around her wrist instead. When the rabbit burst forth from his hiding place nearby, Marley (My great Dane, see above) burst into action and all 105lbs of her was hurtled along behind his 140lbs of adrenalin fueled charge. I was distracted by something else when it happened so it took me a few seconds to react and call for him to stop but by that time she had been dragged about 50-70 yards through the snow and mud. After that incident she demanded i get rid of him but he is still my best friend and she is now another ex-girlfriend.