Submandibular gland pathology (e.g., sialadenitis, sialolithiasis, or benign tumors like pleomorphic adenoma): This is the most likely diagnosis due to the location of the mass under the mandible, which is close to the submandibular gland. The submandibular gland is prone to various pathologies, including infections and tumors, which can present as a mass in this area.

Lymphadenopathy (reactive or infectious): Enlarged lymph nodes in the submandibular region can present as a mass under the mandible. This could be due to a local infection, a systemic infection, or even a reaction to a nearby infection.
Dental or periodontal infection: Infections originating from the teeth or periodontal tissues can spread to the submandibular space, causing swelling or a mass under the mandible.
Benign soft tissue tumors (e.g., lipoma, fibroma): These can occur in the submandibular region and present as a mass.

Malignant tumors (e.g., squamous cell carcinoma, lymphoma): Although less common, malignant tumors can present as a mass under the mandible. Early detection is crucial for treatment and prognosis.
Submandibular gland malignancy: While rare, malignancies of the submandibular gland can occur and would have significant implications for treatment and prognosis.
Parotid gland pathology with extension: Although the parotid gland is located more laterally, large masses or those with significant extension could potentially present under the mandible.

Branchial cleft cysts: These congenital anomalies can present as masses in the neck, including under the mandible, although they are more commonly found anterior to the sternocleidomastoid muscle.
Thyroglossal duct cysts: While typically found in the midline, thyroglossal duct cysts can occasionally present off the midline and could be considered in the differential diagnosis for a mass under the mandible.
Metastatic disease to the submandibular lymph nodes: This would be a rare presentation but could occur with malignancies from other primary sites.