Umbilical Granuloma or Umbilical Cyst: This is the most common diagnosis for a small, firm cyst at the umbilicus, especially if it has been present since birth. These cysts are usually benign and result from remnants of the omphalomesenteric duct or urachus.

Dermoid Cyst: A dermoid cyst is a type of teratoma that can occur at the umbilicus. It is a benign growth that can contain skin, hair, and sometimes teeth. It's a possible diagnosis given the location and description.
Pilonidal Cyst or Sinus: Although more common in the sacrococcygeal area, pilonidal cysts can occur at the umbilicus, especially if there's a history of umbilical discharge or infection.
Sebaceous Cyst: A sebaceous cyst can form at the umbilicus, appearing as a firm, sometimes painful lump under the skin. It's filled with a cheesy, semi-solid material.

Omphalomesenteric Duct Cyst or Fistula: While often considered in the differential for umbilical anomalies, an omphalomesenteric duct cyst or fistula can have serious implications if it connects to the intestine, potentially leading to infection or intestinal obstruction.
Urachal Cyst or Fistula: Similar to the omphalomesenteric duct anomalies, a urachal cyst or fistula can connect the bladder to the umbilicus, posing a risk of urinary tract infections or other complications.

Teratoma: A teratoma is a type of tumor that can contain several different types of tissues, including hair, muscle, and bone. While rare, it's a possibility for a cystic structure at the umbilicus.
Lymphangioma: A lymphangioma is a rare, benign growth of the lymphatic vessels that can appear as a cystic mass. It's less common at the umbilicus but should be considered in the differential diagnosis of a cystic lesion.
Neuroenteric Cyst: This is a rare congenital anomaly that results from an abnormal connection between the intestine and the spinal cord during embryonic development. It's extremely rare to present as an umbilical cyst but is a consideration in complex cases.