Buster drama!

So....Our yard is an acre and most of that is the backyard. So we always let Buster out the back and when he is done he comes back to the door. We don't have a fence but for the past 2 years he has taken care of business and came right back. So Tuesday night before Thanksgiving I let Buster out as usually and waited for him to come back...He was out MAYBE 5 minutes so i started calling him and he came running and crying and ran straight to his bed like he was in trouble. When I got to him, he was sticky and stinky so I put him in the bathtub. That is when I noticed he was bleeding. Another dog had attacked him. Ike was at choir and I was here alone... I was freaking out!!! He cried if I even touched him (this went on for days). I had found one hole on his side, when Ike got here he found another. We had to wait till morning to take him to the vet...let's just say I did not sleep at all! He was very bruised and sore. The picture below is today about a week later it is the two holes and they look so much better.... The thing is we have no idea what dog did this!