I was super nervious when I introduced my Thor (then 12 weeks) to Abby (then 5yo). Both are Great Danes and Abby is dog aggressive. The reason I got a puppy instead of another adult dog is that I knew it would be a problem for Abby and hoped she wouldn't feed threatened by a puppy and I was right. She ran up to him full speed as I held my breath. She slid to a stop, started butt sniffing and all proceeded nicely after that. Thor is 6 years old now and Abby is 11. They have had maybe 2 or 3 "squabbles" their whole life.