Joannes Jonstonus (1603-1675) was scientists who specialized in Flora, Fauna, Medicine, and some additional topics. He does not sound nor is he portrayed like some fantasy author of his generation

Among bears, squirrels, lions, cows, elks, rhinos, and other common animals,
Mr Jonstonus decided to present at least six different types of Unicorns among other strange creatures

It’s very easy to say, that the author was just imagining things and did not really mean that they were real. In his book he did not distinguish these creatures in any way, shape or form. He simply included them along with other “regular animals.”

There are hundreds if not thousands of anomalies indicating genetic experiments & cataclysms (resets) within the past several centuries. I believe this Mr. Jonstonus was depicting things just as he saw

Here’s his book Historiae naturalis de quadrupedibus libri, cum aeneis figuris, Johannes Jonstonus,... concinnavit