Reddit reviews Sea Monsters on Medieval and Renaissance Maps

They were all considered actual animals at the time.

One of the earliest, and definitely the most influential, "monster map" is the 1539 Carta Marina whose monsters were copied by several later mapmakers. It was accompanied by a 15-volume book detailing each animal's name and characteristics. Although they were depicted from descriptions and hearsay, and as such are not recognizable to us, most descriptions correspond to an actual animal and the rest correspond to mythical animals that were at the time believed to be real.

On the Carta Marina, you can recognize some of the monster names, such as "Orca" and "Ros Maruspicsis" (walrus), others have different names today like the Vacca Marina drawn as an actual cow. It's Latin for "sea cow", the description in the book perfectly fits what we today call a bearded seal. Although the fantastic sea creatures are what we think of today, this map also depicts a lot of other creatures such as wolves, reindeer, lynx and eagles, and also specific people like kings and warriors. At the time, it was meant as a documentary image accompanying a non-fiction book.

There are several books written on the map monsters, I recommend this one.

Are you willing to add fantasy creatures? This book might supply some inspiration.