My late Great Dane Bogart was a man and an expert fighter. Once in West Philly I saw quickly dispatch three straight rats as they came out of a sewer. All instinct, all in the genes. He came round lightning fast and bit each one on the back of the neck, thus avoiding their fangs.
With other male dogs, he never ever started a fight, but he was always ready, and always prevailed. He had this GREAT move where he would sweep once massive leg through the front legs of his adversary and literally knock 'em on their backs. He didn't bite them, he'd just loom over them then, and they ALWAYS gave up.
With cool dogs, he was always cool back. Two of his best friends were dobies.
Only twice did I ever see him bested. Once was when he got too near to a calf, purely by happenstance, and the momma cow backed him up big time. The other time a Saint Bernard even bigger than he was (175 pounds, all muscle) got the drop on him, right from behind on his neck. He just froze and acquiesced.