There were four types of dragon in ancient Greek mythology : the serpent Dracones, the marine Cetea, the fire-breathing Chimaera and the she-monster Dracaenae.
The first type of Greek dragon was the Dracon whose name was derived from the Greek words "drakein" and "derkomai" meaning "to see clearly" or "gaze sharply." It was essentially just a giant serpent which was sometimes equipped with rows of sharp teeth, deadly poison or multiple heads. In myth the beast usually guarded a sacred spring, grove or golden treasure. Our own word "dragon" derives from the creature's name.
The second type of dragon was the Cetus or "Sea-Monster." The creature usually featured in myths of a sacrificial princess rescued by a hero.
Cetacea = Dolphins
The third type of dragon was the Chimera, a fire-breathing monster whose form was a hybrid of lion, serpent and goat. Medieval artists used this creature as the template for the Dragon of Saint George.
The fourth type of dragon was the Dracaena or "She-Dragon," a creature with the upper body of a beautiful nymph and the body of a dracon or sea-monster in place of legs. Two of these creatures, Echidna and Ceto, spawned most of the dragons of myth.