Parallelism between the animals found on land and sea was a common theory that was believed in the age of classical antiquity. This basically meant that it was thought that every creature that could be found on earth would have a similar counterpart found in the ocean, and in some further extensions of this theory possibly the heavens above as well.

Pliny the Elder referenced this notion (one he described as vulgar, meaning ‘common’) in Book 9 of his Natural History. He was also surprised at the extent that he found it to be true in many stories, even those that extended to inanimate objects:

Hence it is that the vulgar notion may very possibly be true, that whatever is produced in any other department of Nature, is to be found in the sea as well; while, at the same time, many other productions are there to be found which nowhere else exist. That there are to be found in the sea the forms, not only of terrestrial animals, but of inanimate objects even, is easily to be understood by all who will take the trouble to examine the grape-fish, the sword-fish,the sawfish, and the cucumber-fish, which last so strongly resembles the real cucumber both in colour and in smell. We shall find the less reason then to be surprised to find that in so small an object as a shell-fish the head of the horse is to be seen protruding from the shell.
– Pliny the Elder, The Natural History, 9.1.2

In The Book of Beasts: Being a Translation from a Latin Bestiary of the Twelfth Century translator and editor T.H. White expresses the idea that many of the creature found in these medieval masterpieces were possibly inspired by this notion of parallel creation:

It used to be a common belief that everything on the earth had its counterpart in the sea. The horse and the sea-horse, the dog and the dog-fish, the snake and the eel, the spider and the spider crab: these led the extremists to extend their classifications to the air… What was more, if there there were whales on sea and land, why should there not be men in both? Mermen? And if men, why not kinds of men? Why not bishops for instance?
– T.H. White, The Book of Beasts : Being a Translation from a Latin Bestiary of the Twelfth Century, pg 250-251.

And Bishop fish there were – one was even recorded as captured and taken to the King of Poland himself. After being presented to a group of clergymen, the fish gestured it wished to be released, and so was set free – apparently even making the sign of the cross before it sank back into the ocean.

By the 17th Century this long held belief began to disappear, and the realization that many of these creatures had been named out of similarity, but did not necessarily have any connections outside of those parallels:

THAT all Animals of the Land, are in their kind in the Sea, although received as a principle, is a tenet very questionable, and will admit of restraint.[1] For some in the Sea are not to be matcht by any enquiry at Land, and hold those shapes which terrestrious forms approach not … Though many there be which make out their nominations, as the Hedg-Hog, Sea-serpents and others; yet are there also very many that bear the name of animals at Land, which hold no resemblance in corporal configuration.
– Sir Thomas Browne Pseudodoxia Epidemica III.xxiv

But, somehow, for multiple centuries this fantastical idea persisted, with people looking for analogs to creatures on land above as well as bellow.

Did they find what they were looking for?

Perhaps only the Bishop fish knows the answer…