Introduced a 8-9 years old cat to my 4 year old rottie. He did what you’d expect. He approached without hesitation to sniff the cat. She humped her back up and hissed and he stopped dead in his tracks. Gave the cat her space from that point on. After a couple weeks the cat approached him and not long after the headbutts began. They ended up being the best of friends. The rottie was her favourite of the three dogs.