Merlin & Strider play fight all the time, sounds terrible, looks awful but totally benign. Raven, on the other hand, will periodically take on my Mom's shepard mix, Lady, on one of her frequent visits. It will start over nothing, just walking past each other, and one doesn't like the others look, I guess. I have to break it up, because the other two will decide to step in and then I have a four-way fight to handle. There has been one trip each to the vet for stitches, and one trip to the ER for me for stitches when I did't move fast enough to intervene in the situation. After I got slashed by a stray tooth, I totally lost it and sent everyone cowering into corners. (They were still there when we returned from the ER) Since then there has only been one serious altercation, and I stopped that with a loud "KNOCK IT OFF!!!" and a glare.
Stella