Wait. Old Yeller was not afraid of that wolf.
I don't go for that knowledge in the DNA that the wolf is the superior ones on the food chain. I expect there are plenty of dogs that would face down a wolf.
I know dogs will acquire fears through experience. Look at a dog that survives a shooting. Thunderstorms become quite a terror for them, even for the undisputed alpha dog.
I find it unlikely an adult English Mastiff or similar breeds fear much of anything. They stood up to lions and bears. They may not be smart enough to fear wolves.
If the dog fears something out there, there are other aggressive critters from which to choose. Mountain lions, some bobcats, bears, wolverines and moose are not the most friendly. An event that was not witnessed by the owner can leave a lasting impression.
Still, I have seen dogs that will just not show fear or back down.