I just discovered that elephants do not have scrotal sacs. Why is that interesting? Well, about 2 million years ago, male teenage elephants liked playing a game (we would call) kick in the balls. Those with large, hanging fruits presented a better target. Naturally, they got kicked more. Unfortunately, this led to testicular damage which led to them shooting blanks and being denied the joys of fatherhood. This gave owners of smaller sacs an evolutionary advantage. This, coupled with more accurate kicks eventually led to the balls retreating completely into the body.
Also, they are supposedly descended from aquatic mammals. Those ones had internal balls.