I have a 7 year old dog (Manson). I have had him since he was 6 weeks old. Very good and very well behaved dog. I brought in another dog which is 2 years old (Winston) and he has only been with us for 3 days now. Manson has become slightly aggressive with us lately he bit my boyfriend last night. Manson also has not been very nice to the other dog. Manson has hip dysplasia. Do you think that he is in pain from the dysplasia or do you think that he is upset about the new dog Winston?

I am really concerned. I did read that dogs tend to turn on their owners but it just really depends on how they are raised. He has never been raised to be aggressive, he is a very well behaved dog and always has been.
He had been good with other dogs before but only dogs that he has been around often though. This new dog is one that he never met before. Manson doesn't rough house with Winston. Winston is always trying to sniff Manson and get in Mansons way.

Dogs that are in pain do not tolerate other dogs very well. With the second dog, your first dog (already being in pain) could be in MORE pain because the situation is stressful for a hurting dog with the addition of a new pack member.
You don't raise dogs to be aggressive. You raise them to be social. If you don't do that (between 6 & 12 weeks), then the dog will not be able to handle a change of environment.

He is prolly just trying to make his place in the house known. Dogs are pack animals, and in every pack there is a leader. He is trying to lead your family as "pack leader" or "alpha dog". with time he should get used to the situation, but do not back down. Treat him a a pet, not the leader. YOU are the leader. Make sure to make sure he fells love though, because he prolly feels very rejected.

If the behavior continues, a different solution will be needed. speak to a dog trainer or breeder for help in this case.

good luck

It is likely some combination of pain and stress. Is he being treated for the hip dysplasia? You may want to try to ease his stress by being sure you are maintaining his previous routine before Winston came home. Really you should change nothing for Manson just because there is a new dog in the house. See link for how to introduce a new dog to your resident dog. Good luck.

when bringing a new dog into your home when you have a dog already you should keep them separated for a week to ten days. Either in another room or in a crate so that they can only sniff each other to get used to each other and scents. It makes the transition easier on both dogs.

Most of the time it is just jealous of the other dog, may be in pain if the other dog is allowed to aggravate the older dog, it does take time for the two to get along, just be fair with your time, treats and give your old dog the same amount of time you did before. Always have a dog checked by the vet when there is a temperament change to be safe, you would`nt want your dog in pain.