The guard dog breeds are the gentle but formidable giants of the working dogs. They often go unnoticed. They are not as flashy as the herding dogs and demand very little and there are fewer breeds to pick from.
Original livestock guardian dogs were village and country dogs who accompanied the shepherd and flock on their annual migrations and by doing so helped to guard the flock. Dogs that were detrimental to the well being of the flock were not kept.
Today, in North America, we see guard dog breeds with watered down traits which is a sad thing to witness. Perhaps this is due to our kind nature, smaller holdings, and less sever lifestyle. Nonetheless, we're still asking our dogs to go up against some sever predators so we need to retain the strong guardian and loyalty traits in these breeds.
The guard dog breeds we know today bear the names given them from the countries and regions they originated from. If one were to travel to some of these regions you would find several more than the few breeds listed below. Sheep did not originate in North America and neither did any of the livestock guardian breeds.
The more common dog breeds in North America include several of the white dog breeds such as Maremma, Kuvasz, Akbash, Komondor, and Great Pyrenees.
Other not so common breeds include the Anatolian (becoming more popular), Ovcharka, Pyrenean Mastiff and Sarplaninac.
The reason there are various breeds is because there are various people living in various regions. In very general terms different dog breeds developed because there was always need for specific traits to help get the work done in that locale. Hence the different breeds that show up among the various cultures and regions.
In general and regardless of breed, these dogs have a very independent nature. In other words they are stubborn, which can make them frustrating to deal with. They are intelligent in a thinking and conniving way. So although they may not be the quickest to learn new pet tricks, they will problem solve. They are also very brave dogs who can pack a lot of courage. They will be quick to climb the social ladder and assume the leader position if you don't assume the role yourself.
While we can make these general assumptions on the traits you're likely to encounter in livestock guardian dogs, selecting a guard dog breed is a really a matter of what you need and who you are.
The Book Livestock Protection Dogs, Selection, Care and Training by Orysia Dawydiak and David Sims provides thorough descriptions of the breeds and their working characteristics.