Mediaeval artists were fond of illustrating the maps they drew of the world as they knew it, with fantastical animals. Huge monsters who lived in the deep oceans...fish with wings and sea birds with monstrous beaks and claws.
They drew the pictures based on first- hand accounts of the intrepid explorers who'd set off in leaky wooden sailing boats to travel as far as they could go and hope to come back again...
It must have been incredibly alarming to see a whale for instance...if you were soaked to the skin with salt water and probably suffering from scurvy and sun blisters to suddenly spot a huge whale passing close to the boat you were in must have frightened everyone on board totally witless...easy enough then to exaggerate once you returned home about the size of the animal and the strange calls it made...
But were those early explorers simply telling tall tales or did they really see monstrous creatures...a school of dolphins with their smiley faces would have been commonplace...but did they see other sea animals we are unfamiliar with...did they really see monsters.
The oceans would have been totally unpolluted in 1100 and 1200...probably still unpolluted until 1600...the water was cleaner...conditions were different...no fuel spills from cargo ships, no plastic, no effluent from factories being discharged into the seas...and not many people out and about on the waves either.
So were there animals who no longer come to the surface...were there creatures of the deep who can't live in the surface waters any longer because of widespread pollution so they've retreated to the depths...did those sea faring men really see huge animals with horns on their heads and enormous teeth and great swishing tails...
We think we know the earth and her seas...we imagine we've done all the exploring there is to be done and so we concentrate on outer space instead...but it is thought that among the plants of the Amazon rainforest there is the cure for every disease known to modern man...two years ago an entirely 'new' tribe of people were found...a recent expedition found animals, birds and insects never seen or photographed before...
I see no reason to scorn those explorers of six, seven hundred years ago...if they say they saw monsters, who are we to argue.