Epididymal Cyst: This is the most likely diagnosis given the specific mention of a 2 mm cyst associated with the epididymis. Epididymal cysts are benign, fluid-filled structures that are common and often found incidentally during ultrasound examinations.

Spermatocele: A spermatocele is a cystic structure that contains sperm and is usually located at the head of the epididymis. It's similar to an epididymal cyst but contains sperm, making it a plausible alternative diagnosis.
Epididymal Tubular Ectasia: This condition involves the dilation of the epididymal tubules and can appear as small cystic structures on imaging. It's often associated with obstruction of the epididymis or vas deferens.

Testicular Tumor: Although less likely given the small size and specific location described, it's crucial not to miss a testicular tumor, as some types can be aggressive and require prompt treatment. Ultrasound characteristics and further evaluation would help differentiate.
Epididymitis: Inflammation of the epididymis could potentially present with cystic changes or swelling, and it's essential to consider this diagnosis, especially if there are symptoms of pain or infection.

Adrenal Rest Tumor: These are rare, benign tumors that can occur in the testes or near the epididymis in individuals with congenital adrenal hyperplasia. They are less likely but could be considered in the appropriate clinical context.
Cystic Dysplasia of the Epididymis: A rare congenital anomaly where the epididymis is replaced by multiple cysts. This condition is uncommon and usually identified in childhood or during evaluation for infertility.