Early map makers often recorded on their maps and charts the presence of fabulous beasts, many of which now reveal themselves as identifiable species made grotesque by the embroidered tales of sailors and travelers.
Of the many maps depicting sea monsters, the Carta Marina by Olaus Magnus was the most influential. First printed in 1539, it showed the area of the North Sea filled with dangerous maritime creatures. Versions of Olaus Magnus's monsters populated the seas on charts until the mid-18th century.
Ortelius copied many of these monsters for his map of Iceland, published in editions of his Theatrum Orbis Terrarum. Iceland is shown at large scale, with its fjords, glaciers, and mountains, including an erupting volcano, Mt. Hekla. Polar bears on icebergs can be seen at the top right, while the ocean contains probably the most fantastic collection of sea monsters to be seen on one engraving (shown here).
http://www.library.yale.edu/MapColl/lan36.htm
So generally the monsters turned out be unidentifiable species of the time, with grotesque embellishments by story-telling sailors and travellers. Maps of this nature also sold very well because of peoples natural curiosity.