I had a great Dane, 'Charly' who has since died, he scared Thais even though he was a gentle soul but he didn't like it if a stranger touched me, then a low threatening, grumbling, growl would come from him for a few seconds as a warning. I got over the question of poisoning by getting strangers to throw him some meat over the wall laced with chilly on the inside, after the third time he refused anything that wasn't from his food bowl.