The reason for a lump on the left side flank of a dog behind the ribcage is usually due to a tumor or lipoma. Lipoma is a common fatty deposit that is found underneath the skin.
Lipoma lumps do not need to be removed in most cases unless they interfere with the dog's ability to move and affect its quality of life. If the lump is a tumor then the doctor will need to first perform a biopsy to ensure that it is not cancerous. If the lump is cancerous then the doctor will need to remove it and to also provide other treatment depending on the cancer's stage and type.