I have a Great Dane that is almost 5 years old. He is a wonderful dog but he has been aggressive with my other dogs many times. It got worse the older he got. We had him neutered when he was about 1 1/2 years old which helped until the past few weeks. He has shown some aggression again recently so we are watching him very closely when he is with the other dogs. He has never shown any aggression with my cats. Good luck with Thor.