A wyvern is a legendary, dragon-like creature that has wings, two legs and breathes fire. Its head is like a dragon, its body like a reptile, and its tail is barbed.
There is also a sea-wyvern that has a fish’s tail in place of the barbed one.
The word comes from a 13th century French word wivre, meaning viper. Although the wyvern figures prominently in heraldry from the 17th century, it is first seen in medieval bestiaries, illustrated books that depicted and described various beasts, both real and imaginary.
My first encounter with the word was in Jo Beverley’s Regency romance The Dragon’s Bride, in which the hero, Con Somerford, inherits the Earldom of Wyvern, complete with a monstrous castle that contains a huge statue of a dragon about to deflower a virgin with its barbed tail. A continuation of the story that also involves an Earl of Wyvern is A Shocking Delight. I highly recommend both books.