Early cartographers often included illustrations or descriptions of sea monsters on their maps, in order to warn explorers of their location. Sometimes the drawings were symbolic, such as a picture of a king riding a fish, to denote who ruled that part of the sea. Others were supposed to be literal monsters, but the pictures were created from descriptions by frightened seafarers, often years after the fact. The image shown here is from around 1230 AD, depicting a whale that sailors mistook for an island.
"In the Indian Ocean there are whales which are so large that they seem to be islands. And sometimes because of the soil they have on them plants grow on their backs. Men crossing the sea sometimes land on these whales, which, when they feel the movement of men on them, hurry down into the depths, and so the men are drowned."
The post at BibliOdyssey is a little different direction from usual in that it has illustrations from a new book, Sea Monsters on Medieval and Renaissance Maps. However, the many illustrations are hundreds of years old, which is right in line with the usual fare. Link