The United States National Library of Medicine has a fantastic online exhibit about the history of conjoined twins, beginning in medieval times. Titled "From 'Monsters' to Modern Medical Miracles," the site explores the science and culture of "Siamese" twins. Seem here is a plate from a 1499 book by Jacob Locher, Carmen heroicum de partu monstrifero, "one of the earliest printed illustrations of a set of conjoined twins and the earliest printed work devoted entirely to the subject." From the site section titled "Age of Superstition":
From medieval times through the Enlightenment conjoined twins were viewed as monsters. Their existence simultaneously horrified and amazed the common person. The established medical explanation of the day, from Hippocrates, reasoned that a conjoined twin was simply the result of there being too much seed available at conception for just one child, but not enough for two distinct beings. Even so, popular theories fueled the public's fear and wonder by suggesting that conjoined twins were the result of impure conception or the witnessing of some evil or traumatic event during pregnancy.Link
Books depicting all sorts of monsters, both real and imagined, were extremely popular among the literate during this period. The authors often copied extensively from each other, bringing long told tales with new illustrations to another generation of the fascinated.