Species mentioned: One adder (V. berus) who successfully provokes every knight in Britain and brings about the death of King Arthur.
Source: The ‘Stanzaic Morte Arthur’, the immediate forerunner of Thomas Mallory’s more famous ‘Le Morte dArthur’.
Date of Source: Fourteenth century, probably c.1350.
Highlights: A concerned adder realises that there is a possibility Arthur might reconcile with Mordred, leading to another hundred pages of boring story for poor students to read. It heroically glides (honestly!) from its hiding place and bites someone. This stirs-up a blood bath with 100,000 knights slaughtered, including many of the most angst-ridden ones. Soon afterward the text ends with no chance of a sequel. You’re welcome.
On a more serious ecological note, this text is symptomatic of the antipathy people had for snakes even in the medieval period.