Final answer:
A large uterine mass in an intact female potbellied pig is most commonly a uterine leiomyoma, which is a benign tumor originating from the uterine muscle layer.
Step-by-step explanation:
The likely cause of a large uterine mass in an intact female potbellied pig is most commonly associated with a uterine leiomyoma. Uterine leiomyomas, also known as uterine fibroids, are benign tumors arising from the muscle layer (myometrium) of the uterus and are the most frequently diagnosed type of uterine tumor in both humans and animals. Uterine adenocarcinoma, a malignant tumor that originates from the lining of the uterus, is less common but still possible. Uterine fibrosarcoma and uterine lymphoma are other types of uterine tumors but are rare compared with leiomyomas.