Thyroid Nodule: The most common cause of a hard nodulate on the anterior aspect of the neck is a thyroid nodule. These can be benign (such as adenomas) or malignant (such as papillary thyroid carcinoma). The fixed nature suggests it could be adherent to surrounding structures, which raises concern for malignancy.

Lymphadenopathy: Enlarged lymph nodes in the neck can present as hard nodules. Causes can range from infectious (e.g., tuberculosis) to malignant (e.g., lymphoma or metastatic disease).
Branchial Cleft Cyst: Although typically presenting in childhood, these congenital anomalies can sometimes be diagnosed in adults, especially if they become infected or enlarged.
Salivary Gland Tumors: Tumors of the submandibular gland, which is located under the jaw, can sometimes present as a hard nodule in the anterior neck.

Metastatic Disease to the Neck: A hard, fixed nodule could be a metastasis from a primary cancer elsewhere in the body (e.g., lung, breast, or gastrointestinal tract). Missing this diagnosis could significantly delay appropriate treatment.
Anaplastic Thyroid Cancer: Although rare, this aggressive form of thyroid cancer can present as a rapidly enlarging, hard nodule in the neck and has a poor prognosis if not promptly treated.
Lymphoma: Both Hodgkin and non-Hodgkin lymphoma can present with lymphadenopathy in the neck, which may feel like hard nodules.

Carotid Body Tumor: A rare, usually benign tumor of the carotid body, which can present as a hard, pulsatile mass in the neck.
Paraganglioma: Similar to carotid body tumors, these are rare neuroendocrine neoplasms that can occur in the head and neck region.
Castleman Disease: A rare disorder that can cause lymphadenopathy, which might be felt as hard nodules in the neck, among other symptoms.