The Wyvern of Wye was a dragon-like beast that was terrorizing the West Country of England, until the brave knight Sir Cadogan rode forth. Unfortunately the creature ate his horse and broke his wand, but Cadogan found a fat gray pony and jumped on it before charging back at the Wyvern. With his broken wand, he split the creature’s tongue, causing it to explode (Pm). Sir Cadogan’s fight with the Wyvern inspired a saying in the Wizarding World “I’ll take Cadogan’s pony,” which means to make the best of a horrible situation (Pm).
"Wyvern" - Anglo-French wivre, from Old North French form of Old French guivre "snake," from Latin vipera "viper" (see viper). In heraldry, a winged dragon with eagle's feet and a serpent's barbed tail.