Watchful, aloof, imposing, and intimidating: The ancient Tibetan Mastiff is the guardian dog supreme. These densely coated giants are mellow and calm around the house, sweetly devoted to family, and aloof and territorial with strangers. Coming face to face with this ancient behemoth, an intruder up to no good will likely move on to easier pickings. TMs can stand 26 inches at the shoulder and weigh well over 100 pounds. It's impossible to discuss this breed without leaning on words like 'powerful,' 'muscular,' massive,' and 'substantial.' And yet, TMs are quite light-footed and will meet a perceived threat with surprising agility. The broad head, with its high-set, V-shaped ears and expressive brown eyes, projects a noble, sagacious expression.