Sounds like what is going on in my house at the moment. I've had Max since last year, he is 5 years old. I got Lily last week, she is 6 months. Max has never been really social, but he seems to get along with females who have the right personality. He was not happy when we brought Lily home. She is not potty trained yet, so she spent her first days mostly in her xpen. The first day, Max would go up to the xpen and bark constantly. When I took her out, he growled and barked. He would get treats and praise when he didn't. That took a couple of days. He tolerates her now. She chases him and wants to play but he's not having any part of it. Its been a little over a week and its getting better each day.
I'm always sure to give him extra attention and help reassure him that he's not being replaced. Although they are fed at the same time, I put his bowl down first. He gets his treat first..stuff like that. I take them outside to potty together. I think these things have helped us.
I got lots of help and encouragement from a couple of YT members and I am so grateful!
