I brought an intact male boxer home 3 weeks ago today. He was neutered on a Tuesday 2 weeks ago. He has been getting along amazing with my 4 boxers. 3 of mine are males all neutered and 1 is a female spayed. It was until Friday where they had an altercation. My foster is named Otto and my male who is having issues with him is Capone. They had a fight while I was at work. They each received a puncture wound. Then yesterday while my husband was home with them they had 2 fights. When I got home I could tell Otto was on guard and Capone was trying to avoid him. As soon as Otto would see Capone he would perk up and run at him to attack. We have obviously kept them separate however I'm not sure why this has happened all of a sudden. I understand Otto may have raging hormones still. He also was very thin and is starting to gain weight so I'm not sure if he is feeling good enough to challenge Capone. It seems as he is holding a grudge against Capone. They are fine seeing each other through their crates and even lay by each other when 1 is in a crate but once out it is game on. Any Ideas? Ottos is around 8-9 and Capone is 7. Otto is a foster btw however i do not want to return him.