There are honest mistakes made by imaginative sailors who’ve been out to sea too long, and then there are “Jenny Hanivers.” Ulisse Aldrovandi, who published this image in 1640, described it as “basilicus ex raia,” indicating an awareness of the illegitimacy of the creature. Neither mermaids nor basilisks, the carcasses of dried rays or skates were mutilated to resemble human-fish hybrids, lacquered, and sold to tourists. 
Follow #bhlMonstersRreal on Twitter and Facebook to learn more about the real animals that many legendary monsters are based on. See more historic monster illustrations from the Biodiversity Heritage Library.

GIF based on Aldrovandi, Ulisse. Serpentum, et draconum historiæ libri duo. 1640.