From Slate: In the early days of mapmaking, the seas were full of monsters. Close to port or in well-explored shipping lanes, stout frigates and galleons were depicted in full sail, but farther out, a remarkable diversity of sea serpents and other bizarre creatures ploughed the waves. On land as well, uncharted territories were generously populated with legendary figures both pagan and religious, both human and … clearly otherwise.
The weird bestiary at the edges of maps was in large part an artistic decision, a chance for cartographers to fill in ugly white spaces of the still-unexplored Earth and to stretch their creative wings. (Engraving awesome, foam-spouting behemoths must have been a nice break from tracing the coast of Mexico for the umpteenth time.) But they also served as a reminder of the very real dangers faced by the explorers of the day. No one knew what was out there, and many who left didn't come back.