"All of the species depicted in Louis Renard’s _Poissons, ecrevisses et crabes_ received some level of artistic embellishment – and approximately 9% are completely fantastical. If that’s not strange enough, the work is produced by a man who identified himself as a 'secret agent on behalf of the British Crown' and contains a portrait of a mermaid."

https://blog.biodiversitylibrary.org/2016/08/renards-book-of-fantastical-fish.html