Effective guard dogs should be loyal, intelligent and territorial, but not necessarily aggressive. Some breeds work well because they are large and have intimidating features, as others have deep barks and growls that warn potential intruders away just with the sound alone. There are several dog breeds that were bred specifically for guard and protection duties, while other breeds were brought to it because of their fearlessness and natural protective instincts.
Initially bred by a tax collector to protect him during his collecting duties, the Doberman is a medium to medium-large size dog with dark coloring, a pointed face, and often cropped ears and a docked tail. It is intelligent and highly trainable as well as fearless of intruders. Because of its natural inclinations to protect and its loyalty to its owners, it is often used as a police dog, search dog and guide dog for the blind.
Bred initially as herding dogs, German Shepherds are large and wolf-like in their appearance, with slopping backs and often tan and black coloring. They are the most popular breed for both police and military canine units as they are intelligent and easy to train. They are good with children but can become overly territorial and even aggressive if their behavior is not monitored and enforced regularly.
Similar in coloring to Dobermans, Rottweilers have thicker builds and blockier head shapes, though they will often also have docked tails. They were originally bred to herd and guard cattle as well as protect cattle workers. They are fearless but strong minded, good with children and also highly trainable because they are loyal and eager to please their owners.
Both Thai Ridgebacks (from Thailand) and Rhodesian Ridgebacks (from Africa) work well as guard dogs, with their large size and intimidating statures. Thai Ridgebacks were originally bred to stand guard over homes for peasants while they were at work, and though not as easy to train as some other breeds, are very territorial and loyal. Rhodesian Ridgebacks are good with children but not naturally obedient, though they do want to make their owners happy. They are highly intelligent and reserved with strangers, which makes them protective toward their home and family.
Initially bred as guard dogs for butcher's carts and shops, Giant Schnauzers are larger versions of the standard Schnauzer. They have long beards and bushy "eyebrows," and often docked tails and cropped ears. They are very alert and dependable, as well as being good with children and naturally territorial. While trainable, they can easily become bored if not given enough stimuli during the day and may need training reiterated on a regular basis.