when i got brandi zac didnt like her he would growl and snap at her so i tried giving him more of my time but soon noticed it made him worse,if he was on my lap and she got on he would go for her so instead of putting her down and cuddleing him i pushed him down everytime he did it, it worked they now get on really well,i found it worked better to not take any notice of him when he was playing up and he soon realized to get my attenition he had to get on with her. when i got annie her and brandi didnt get on they had a couple of fights so i tried not taking notice but this time it didnt work so when they didnt get on i told them both off and then gave them time out,that worked and now they play with each other all day.so it depends on the dog to what you can do about it. if the older dog is on your lap and the new one gets on and your older one growls put your older dog down but dont say anything,my guess he will learn if hes bad he gets nothing from mummy but if hes good mummy gives him a cuddle.
