Both my old rescue GSD and rescue Boxer x GSD have defended me on a couple of occasions. No background for the GSD as we found her half dead in a local park, so no idea if she had had some degree of protection training, but if you grabbed the back of her collar in a certain way she would rear up and go mad. She ahem ‘politely’ told a guy who was very drunk and being inappropriate towards me at a friend’s party to bugger off and he didn’t stop to think twice. She never used her teeth, but then she didn’t need to as she looked pretty intimidating even when she was perfectly calm and happy (old-style long-haired GSD and massive, especially for a female).
My Boxer x GSD once chased off a man who was actually trying to steal him and then did it again when someone (probably the same guy) broke into our house (the police thought a local dog fighting ring wanted him). When we got home he was sitting in the kitchen waiting for us. He’s shredded his feet on broken glass and splinters where they had kicked in the gate and smashed the back door. My neighbours said he ran over it all and chased them down the street. Our other dog (Whippet x Sheltie) just legged it and came home a couple of hours later.
Collie x Belgian Terv, yes, I think she would have done if necessary. She was very loving and affectionate, but also an independent thinker (which made for lots of, erm, fun training her). She was also really steady, but very concerned with keeping her family together on walks and constantly scanning around us. She completely ignored other dogs and people out and about, unless they broke her imaginary perimeter, then she would just quietly put herself between us and them and head them off. If someone came up to speak to me while I was walking her alone she would shift from my side and sit on my feet, putting herself calmly between me and them. Beautiful dog through and through. Never growled or barked at anyone in her life, but I do think she would have defended any one of her family members if push came to shove.
My two Lurchers were very different. One (the biggest one) was a complete wuss and would hide behind me, but the other, who was the most laid back and gentle dog in the world, was also really emotionally intelligent. If he sensed I was worried about someone when we were out walking his whole demeanour changed and he would subtly move between me and the other person. He never had cause to defend me, but I am sure he would have done had the situation called for it.
I now have a toy breed and I am pretty sure he would defend me. He hates it if anyone play-fights in the house and will put himself between the two people and bark to tell them off. Although he never does anything more physical than that, ie no teeth. I strongly suspect if someone was genuinely attacking me that he might actually be more serious about seeing them off though.
One of my doubts about getting a small breed dog after having large breeds for years was that I wouldn’t feel as safe walking them, but actually I think he would make such a racket if anyone tried anything that he’d probably be quite a good deterrent.