Medusa

The Medusa is a snake-haired humanoid seen in the desert of Llewdor. Whoever looked at her turned into a stone. Alexander had caught her from the corner of his eyes several time, but luckily never looked at her directly. Later Alexander managed to turn herself into a stone with the help of a mirror (as he remembered reading that Medusa had been killed once by a hero using a polished shield).

There are actually many "Medusas" that live out in the desert, and which have turned many people into statues. Though the Medusa that Alexander defeated was apparently the leader of her race, and the one from which her race took its name.

Real world
Medusa was a gorgon from the Greek mythology. Like Alexander, Perseus killed her by looking at her from a mirror. He cut her head and put it on his shield so that he could turn his enemies into stone.

While Gorgon would be more accurate term to use, "Medusas" is a plural term for the creatures within Daventry.