Species description
In the 13th century, the didactic text Konungs skuggsjá was written. It had many descriptions of whales, seals, and monsters of the Icelandic seas. The descriptions were short, and often contained errors. The descriptions include those of a mermaid, and of a large sea monster called Hafgufa. At first, the author did not want it,  because he thought it was too big, but felt it was important enough to be included in his descriptions. Today, we know that the Hafgufa does not exist.