Benign lymphadenopathy: This is often due to a viral or bacterial infection, and the lymph nodes may become enlarged and fluctuant as they fill with pus or other inflammatory material. The body's immune response to the infection can cause the lymph nodes to become tender and swollen.

Tuberculous lymphadenitis: Also known as scrofula, this condition is caused by Mycobacterium tuberculosis and can lead to the formation of fluctuant masses in the neck due to the infection of lymph nodes.
Reactive lymphadenopathy: This condition occurs in response to an infection or inflammation elsewhere in the body, leading to lymph node enlargement. The fluctuance may be due to the accumulation of inflammatory cells and fluid.
Branchial cleft cyst: A congenital anomaly that can present as a fluctuant mass in the lateral neck, often becoming apparent during infections when the cyst becomes inflamed and filled with pus.

Malignant lymphoma: Although less common, lymphomas such as Hodgkin's or non-Hodgkin's lymphoma can present with lymph node enlargement, including fluctuance if there is necrosis or infection within the tumor.
Metastatic carcinoma: Cancer from other parts of the body can metastasize to the lymph nodes in the neck, potentially causing fluctuant masses if the metastases become necrotic or infected.
Abscess: A collection of pus in the tissue, which can occur in the context of a lymph node or in the soft tissues of the neck, presenting as a fluctuant mass.

Kimura disease: A rare, chronic inflammatory disorder that can cause lymphadenopathy and the formation of fluctuant masses in the head and neck region, often accompanied by eosinophilia.
Kikuchi-Fujimoto disease: A rare condition of unknown etiology that can cause lymphadenopathy, often in the neck, and may present with fluctuance due to necrotic lymph nodes.
Castleman disease: A rare disorder that involves an overgrowth of cells in the lymphatic system, which can lead to lymph node enlargement and potentially fluctuant masses in the neck.