The large, fire-breathing reptile common in European folklore is the dragon.
European dragons became especially recognizable during and after the Early Middle Ages. They were commonly imagined as enormous, scaly, horned, winged reptiles that lived in caves, mountains, castles, or underground lairs. Many stories made them guardians of treasure and opponents for heroic figures such as Saint George, Sigurd, or Beowulf.
The word comes through Latin draco from the Greek drakōn, a term that originally referred more generally to a large serpent. Medieval European art and bestiaries gradually helped establish the familiar Western image of a four-legged dragon with wings, claws, a long tail, and fiery breath.
A common mix-up is the wyvern: in later heraldic tradition, a wyvern usually has two legs, while a dragon has four. A basilisk and cockatrice are also legendary reptiles, but their defining traits involve deadly looks, venom, or unusual animal features rather than the broad fire-breathing dragon tradition.