Last sunday happenen something totally unexpected. Over two months ago I hosted my brother´s golden retriever (a year old female) on my house for more than a month while he was moving back from Argentina
She became best friend with Misha (at that time only girl), played all day slept together in the same house, etc. It was after she returned home to my brother´s that I decided to get Homero to keep Misha company.
Last sunday I took both my dogs to visit my brother´s house but the welcome was not what I expected. The golden immediately became aggressive towards Homero (he´s a 4 month old puppy) and attacked him - no harm done just scared the poor kid.
She then started growling at Misha constantly, first she ignored her but then after a while decided she´d had too much and fought back. We separated both girls on time with no harm.
What puzzles me is that Misha is very used to visiting her friends, all adult females (my other brother´s Bernese, and two other yellow labs from my mountain biking friends) and has never gotten into a fight, and now she got into a fight with her previous best friend...
I could understand if it had been a strange dog (since we were invading her territory) but two dogs that lived together for more than a month?
My other brother´s Bernese has always welcomed us when we visit her and she received Homero last week with "open paws"...
Any thoughts? should I avoid getting the dogs together again?
I think I would try to take them somewhere that is neutral to all of them and see how she reacts. It sounds like more of an issue with your brothers dog than yours. Maybe she's being territorial over HER house and is fine away from home.