Testicular Torsion with Atrophy: The ultrasound findings of an atrophic, possibly fibrotic right testicular gland with reduced mobility and no documented blood flow are highly suggestive of a previous testicular torsion that has led to testicular atrophy. The presence of a large left hydrocele is a separate finding but does not directly relate to the right testicular pathology.

Undescended Testis with Atrophy: Given the mobility between the inguinal canal and scrotum, this could indicate an undescended testis that has undergone atrophy, possibly due to inadequate blood supply or previous torsion.
Testicular Regression Syndrome: This condition involves the involution of a testis, which could present with similar ultrasound findings, including atrophy and reduced blood flow.

Intermittent Testicular Torsion: Although the current ultrasound shows no blood flow, intermittent torsion could lead to periods of ischemia followed by reperfusion, causing chronic damage. This diagnosis is critical to consider because timely intervention can save the testicle.
Malignancy: Although rare in infants, testicular malignancies can present with testicular enlargement or atrophy. Given the age and ultrasound findings, this is less likely but should not be entirely ruled out without further investigation.

Testicular Infarction due to Other Causes: Conditions such as vasculitis or trauma could lead to testicular infarction and atrophy. These are less common but should be considered in the differential diagnosis, especially if other systemic symptoms are present.
Congenital Anomalies: Certain congenital anomalies, such as testicular dysgenesis, could potentially present with atrophic testes. However, these would typically be identified prenatally or at birth.