I have a 5 yr old chihuahua that has lump on the right side of his rib-cage and if you touch it hurts him what could this be?

There are several possible causes for a lump on a dog's rib cage, including:

- Lipoma: A lipoma is a benign, fatty tumor that is common in dogs. Lipomas are usually soft and movable, and they do not typically cause pain.

- Histiocytoma: A histiocytoma is a benign tumor of the skin that is common in young dogs. Histiocytomas are usually red, raised bumps that can occur anywhere on the body. They are typically not painful, but they can itch.

- Mast cell tumor: A mast cell tumor is a type of cancer that can occur anywhere on the body, but it is most common on the skin and in the digestive tract. Mast cell tumors can be red, purple, or brown, and they can be firm or soft. They can be painful, especially if they are located in an area where they are rubbed or bumped.

- Osteosarcoma: Osteosarcoma is a type of bone cancer that is most common in large breed dogs. Osteosarcomas can occur anywhere in the body, but they are most common in the long bones, such as the femur and tibia. Osteosarcomas can be painful and can cause lameness.

- Hemangiosarcoma: Hemangiosarcoma is a type of cancer that arises from the cells that line the blood vessels. Hemangiosarcomas can occur anywhere in the body, but they are most common in the spleen, liver, and heart. Hemangiosarcomas can be painful and can cause bleeding.

If your dog has a lump on his rib cage, it is important to take him to the veterinarian for diagnosis. The veterinarian will be able to determine the cause of the lump and recommend the best treatment.