Simple SummaryThe complete duplication of the caudal vena cava (CVC) in a 10-year-old male cat is presented based on a vascular corrosion cast. The literature search shows that views on the development of the pre-hepatic part of the CVC both in domestic mammals and of the inferior vena cava in humans vary considerably. Recent research using 3D reconstructions has brought new insight into the meaning of the caudal cardinal veins into this process. The highest incidence of this anomaly was found in rats, cats, and small breeds of dogs (3–27%), while in many more examined humans it was lower, estimated at 0.2–3.0%. In addition, the concomitant pathologies vary—in small domestic mammals the presence of a double CVC was often connected with the ureteral malposition, while in young adults the double inferior vena cava increased the risk of deep venous thrombosis. In 50% of bilateral venous thrombosis cases, coincidental congenital defects of the cardiovascular system were rare. We believe that the presented case and literature review can contribute to better knowledge regarding the deep abdominal veins—their development, variability, concomitant pathologies, and accurate diagnosis.AbstractPrecise knowledge of the species-/breed-specific anatomy is important for accurate diagnosis and treatment. Existing literature has also been increasing in accordance with the growing demands of biomedical research, wherein mammals, including cats, have been used worldwide. Based on a vascular corrosion cast, complete duplication of the caudal vena cava (dCVC) was accidentally found in a 10-year-old male cat. The two separate symmetric veins corresponding to two caudal venae cavae cranially directed on both sides of the aorta; their first tributaries were the duplicated right and left deep circumflex iliac veins, and the median sacral vein ended in the right common iliac vein. At the L4 vertebra level, the left caudal vena cava crossed the aorta ventrally. It united with the right CVC immediately above the renal veins at the level of the cranial mesenteric artery (L2–L3). Embryologic knowledge is essential to understand the differences between the CVC variants in domestic mammals and the inferior vena cava in humans. However, views regarding the post-hepatic segment of the CVC during development vary considerably. Therefore, our case report also includes a summary of the CVC developmental theories and their clinical impact. We believe that this case and literature review contribute to the knowledge regarding the deep abdominal veins’ variability, concomitant pathologies, and accurate diagnosis and surgery. Additionally, the latest robust studies demonstrating the exclusive participation of the caudal cardinal veins in the CVC development are discussed.