The Drakones were named after the Greek 'drakein' and 'derkomai, meaning 'to see clearly' or 'gaze sharply'. These were guardians, usually of wells and springs, groves, Gods, or treasure. As guardians, they were usually equipped with sharp fangs, deadly poison and/or multiple heads. In essence, they were however seen as giant snakes which--and this is wholly a personal observation--makes sense when most protective and purifying Theoi were depicted as snakes.
Some examples of the Drakones are the Drakon Hesperios (Hesperian Drakon), who guarded the golden apples in the grove of the Hesperides; the Hydra, most famous of all mythological drakons, who had nine, regenerating, heads which grew back in pairs when cut off; and the Drakon Ismenios, who guarded the sacred spring of Ares near Thebes and was slain by Kadmos. The ancient Hellenes also believed that remote, unexplored corners of the earth housed a variety of Drakones, which could be found in Aethiopia, the hills and mountains of India, and in central Anatolia.
The second type were the Ketea, sea-monsters. These resembled snakes, again, but did no guard anything. These were destroyers, usually sent by the Gods in punishment. Cetus, the sea monster sent to ravage the coasts of Aethiopia after a prideful boast by Queen Cassiopeia. Cetus could only be appeased by sacrificing Androméda to it. According to the ancient Hellenes, the Ketea had real-life counterparts as well, in the deep Indian oceans, and in the form of the Scolopendra. Aelian, Hellenic natural historian from the second century AD describes them as follows in his 'On Animals':
"Now in the course of examining and investigating these subjects and what bears upon them, to the utmost limit, with all the zeal that I could command, I have ascertained that the Skolopendra is a Ketos (Sea-Monster), and of Sea-Monsters it is the biggest, and if cast up on the shore no one would have the courage to look at it. And those who are expert in marine matters say that they have seen them floating and that they extend the whole of their head above the sea, exposing hairs of immense length protruding from their nostrils, and the tail is flat and resembles that of a crayfish. And at times the rest of their body is to be seen floating on the surface, and its bulk is comparable to a full-sized trireme. And they swim with numerous feet in line on either side as though they were rowing themselves (though the expression is somewhat harsh) with tholepins hung alongside. So those who have experience in these matters say that the surge corresponds with a gentle murmur, and their statement convinces me." [13.23]
"On first deciphering the fatal message, he ordered Bellerephon to kill the monstrous Chimaera, spawned by gods and not men, that had a lion’s head, goat’s body and serpent’s tail, and breathed out deadly blasts of scorching fire. But Bellerephon slew her, guided by the gods." [VI:119-211]
The Drakaena were hybrids as well, usually with the upper body of a beautiful nymph, and the lower body of a drakon or sea-monster. Most often, these were parents to the above--a necessary evolutionary step. Famous examples include the Goddess Keto, who spawned the Hesperian Drakon, Ekhidna, who was married to the serpent-giant Typhôeus and spawned most of the dragons and monsters of myth, and Skilla, who did not spawn anyone, but was the she-dragon who haunted the Straits of Messina, snapping up sailors from ships--as Odysseus discovered.
These are the four types of drakones the ancient Hellenes identified, some mythological, some actually living in those days. Hopefully this will clear up the confusion of dragons in ancient Hellas and Hellenic myth once and for all.
Image source: Hydra, Cetus, Khimaira, Skilla.