Evil Merchild Suckling (Royal Collection, London)
This portrait is curious for a number of reasons foremost among them establishing the number of breasts inherent a mermaid, but it also establishes the existence of male merbabies, which in this litter were predominant. The chap on mom's right shoulder is feeding vociferously, and clearly male, and obviously trouble.
Nope, you were wrong, it's five breasts!
Giulio Romano is one of the few Renaissance painters mentioned by Shakespeare. In A Winter's Tale the figure of Queen Hermione is compared to the sculpting of "that rare Italian Master, Julio Romano." 
A Mermaid Feeding Her Young 1520-40, Workshop of Romano, Royal Collection, London (image from Wikimedia)
