We have a mixed male chihuahua who is approximately 10 years old that we adopted with a brother of his. He will growl at his brother if he comes near him(very rarely does it develop into a physical fight). If he is laying on the couch or in front of the heater and his brother comes near he will growl and begrudgingly move to another area. I can't understand why he has such animosity towards him since they've been together since birth. We have a male mini shitzu he acts aggressively towards also as well as 2 newly adopted kittens(4 months old now) that he acts the same with. Our only dog he isn't aggressive towards is our 5 year old fixed female chihuahua. I don't understand it since the others are aggressive to him. Every time I try to do a search for this type of behaviour, it talks about introducing a new dog to the pack Every time I try to do a search for this type of behaviour, it talks about introducing a new dog to the pack but none of these pets are new to each other. I won't say that they never get physical, but 99% of the time they do not. If someone has any insight on this type of manner we would greatly appreciate their advice or suggestions.