Hematoma: Given the patient's anticoagulated status, a hematoma is the most likely diagnosis. Anticoagulation increases the risk of bleeding, and a mass on the lateral calf could easily be a collection of blood that has accumulated due to a bleed.

Lipoma: A common benign tumor made of fat tissue, lipomas can occur almost anywhere on the body, including the calf. They are usually soft, movable, and painless.
Cyst: Various types of cysts, such as epidermoid or sebaceous cysts, can occur on the skin and subcutaneous tissues, including the calf area.
Abscess: An abscess is a collection of pus that has built up within the tissue of the body, indicating a local infection. It can present as a painful mass.

Deep Vein Thrombosis (DVT): Although anticoagulation is a treatment for DVT, it's crucial not to miss this diagnosis, especially if the patient presents with a new or worsening mass, as it could indicate a complication or failure of anticoagulation therapy.
Soft Tissue Sarcoma: Though rare, soft tissue sarcomas can present as a mass in the calf. Given the potential for severe outcomes if missed, it's essential to consider this in the differential diagnosis.
Compartment Syndrome: This is a condition where pressure within the muscles builds to dangerous levels, decreasing blood flow. It can present with pain, swelling, and sometimes a palpable mass, and is a medical emergency.

Pseudotumor (e.g., due to chronic hematomas or other conditions): In some cases, chronic conditions can lead to the formation of masses that mimic tumors but are not neoplastic.
Vascular Malformations: Abnormal formations of blood vessels can sometimes present as a mass, especially if they are large or have bled.
Neurofibroma: A type of nerve tissue tumor that can occur anywhere in the body, including the calf, though it's less common in this location.