A 13-year-old, 5.6-kg castrated-male Maltese was presented for reverse sneezing. A dome-shaped round mass abutting diaphragm was incidentally found ventral to caudal vena cava, which had the same echogenicity and density as that of the liver during ultrasonography and computed tomography, showing isoattenuation with a contrast study. Vascular distribution was identified throughout the mass. A caval foramen hernia (CFH) was diagnosed tentatively, followed by a herniorrhaphy and splenectomy of the chronically congested spleen. The patient had been doing well for 5-month postoperative but died because of aspiration pneumonia. CFH is an extremely rare condition, requiring surgery due to compression of the vena cava. It should be considered as a differential diagnosis when intrathoracic, mass-like lesions are identified near the diaphragm.