how old is your dog? From the pic it reminds me of a mass that one of my shepherds developed, he was 12 at the time. It was on his side toward the lower ribs. The vet did X-rays and aspersion which came back probable osteocondrosarcoma a malignant bone cancer. It did not appear painful to him and when I first noticed it it was small, he would fall sometimes and at first I thought he'd fallen and hurt his rib. Definitely I would take him to the vet. It did get larger, the protrusion looked similar to how that appears in the picture, but I can't tell if that is perhaps his shoulder in the angle? At his age we did not opt for surgery, we kept him comfortable until we knew it was time. the lump itself never seemed to be painful to him, he ate well and seemed content. hope the best for you.