It is not uncommon for elderly dogs to develop multiple lipomas, which feel like soft, moveable lumps under the skin. While lipomas are not life-threatening, they can interfere with movement if they become large enough, and internal lipomas may compress internal organs. Lameness. Osteosarcoma is the most common bone cancer in dogs. While tumors usually occur in the long bones of the limbs, osteosarcoma can affect any bone.