A wyvern is a huge flying lizard with a poisonous stinger in its tail. While only a distant cousin of true dragons, wyverns' tendency to hunt in packs makes them dangerous foes. A wyvern’s body is fifteen feet long, and dark brown to gray; half that length is tail. Its wingspan is about twenty feet.
Vox Machina encountered several wyverns being ridden by bandits in "Skyward". In a humorous turn of events, one of the flying beasts was polymorphed into a harmless rabbit, and consequently plummeted thousands of feet into the Ozmit Sea below.
Wyverns were a common element of Thordak's forces, usually used as mounts by the lizardfolk that served him.