Lipoma: A benign tumor composed of fat tissue, which can feel like a hard knot and is common in areas like the shoulder. It's usually movable and painless.

Cyst or Abscess: A fluid-filled lump that can form on the skin, sometimes due to infection. It might feel hard, especially if it's an abscess that has formed a thick wall around the infection site.
Fibroma: A benign tumor of fibrous or connective tissue. It can present as a hard lump and is relatively common.
Bursitis: Inflammation of a bursa, which are fluid-filled sacs that cushion bones, tendons, and muscles near joints. It can cause a lump or swelling on the shoulder.

Metastatic Cancer: Although rare, cancer that has spread to the skin or soft tissues of the shoulder can present as a hard lump. It's crucial to consider this in the differential diagnosis, especially in patients with a history of cancer.
Infected Sebaceous Cyst: While not typically deadly, an infected sebaceous cyst can lead to serious complications if not treated promptly. It can present as a hard, painful lump.
Soft Tissue Sarcoma: A rare type of cancer that begins in the soft tissues, such as muscles, fat, and blood vessels. Early detection is critical for effective treatment.

Dermatofibroma: A rare, benign skin growth that can feel hard and is often found on the arms and legs but can occur on the shoulder.
Pilomatrixoma: A rare, benign tumor of the skin that arises from the hair follicles. It can present as a hard, mobile nodule.
Osteochondroma: A type of benign bone tumor that can occur near the growth plates. While it's more common in other areas, it could potentially occur near the shoulder, presenting as a hard lump.