I really think you have to start a guard dog(of any kind) off as a small (8 weeks old or younger ) puppy in with the flock, goats whatever ou have.
Great Pyrenees is a very common breed to guard animals Bred just for that. but still have to be started really young with the animals they will guard.
They are BIG 100 lbs or more and calm but watchful of attacks by preditors on foot or above...Also long fur and can take the cold weather while outside on guard.
I once saw 2 down my road guarding baby goats and they were on alert mode standing ears pricked.. I had only seen them as i passed by laying around sleeping but when i looked up there was an eagle circleing. That is a dog that nobody would mess with either..