Two Bestiary Sea Monsters

Year: 1638
Scientist: Ulisse Aldrovandi
Originally published in: De Piscibus
Now appears in: "Monk Seals in Post-Classical History" by William Johnson in Mededelingen No. 39
Like Conrad Gesner, Aldrovandi passed along his share of misinformation. In published books, misconceptions could multiply because many artists were illiterate. As a result, illustrations didn't always match the written descriptions they accompanied. It's hard to say what's more remarkable about this serpentine sea monster: it's precise aim in dousing a seal with a waterspout from its own head, or its ability to wriggle on the water's surface. Either way, the turtle observing the spectacle appears entertained.

After examining the depiction and doing what research I can on the depiction, I believe this is showing one of the big moray eels Bernard Heuvelmans spoke of as living in the Mediterranean,  and I think it is probably about the only reasonably accurate depiction we would have of such a creature. The "Vertical undulation" as shown is the belief of most people creatures whether or not it is actually true. Against this possibility are two other objections which come to mind: the first is that eels cannot "Spout Water" as a whale does. True but they can actually spit real water out of the mouth when hauled up on land and that is a possible cause of confusion. The second is that the "Camoflage" colouration Heuvelmans ascribes to this creature is not shown. And to that the only answer would possibly be that the creature should have such markings but the artist chose to make his job simpler by not indicating the pattern.

Year: 1635
Scientist/artist: Juan Eusebio Nieremberg
Originally published in: Historia Naturae
Now appears in: The Science of Describing by Brian W. Ogilvie
Gesner suspected that the walrus (which he called "rosmarus") was the same as another creature known as "morss piscis." That was an accomplishment, considering how different they looked. This especially fuzzy, scrappy picture was likely made from a dried skin. Poorly preserved specimens and confusing illustrations meant that the two animals weren't recognized as the same thing until the end of the 17th century. Nieremberg published this illustration in a book about odd creatures, most of them from the New World. A similar looking animal also appeared in an engraving of the naturalist Ferrante Imperato's museum.

My understanding is that the caption means generically "Marine Fish" and is no clue to the identity. I think this is one of those Master-Otters caught travelling out at sea, and the illustration ois actually taken after the pelt of one such creature. If it is of a length comparable to a walrus (which it does not resemble in any other way), it might be 10-12 feet long. And the flaring out of the tail is probably reasonable enough given the limitated understanding of the illustrator. I imagine the "Streamers" on the bottom toward the rear indicate where the hind limbs would be and probably the hide of the hind legs was damaged either by the removal of the skin, by poor subsequent handling, or possibly by curious individuals taking samples of the hide off, leaving the tattered appearance. This is definitely a very hairy sea-beast, and that narrows down the possibilities considerably.

Both of these engravings and the descriptions are from
http://www.strangescience.net/stsea2.htm