In A Nutshell
The dragon is a legendary creature that is found in mythologies and folklore around the world, and it appears in a variety of forms. Wyrms are typically British and Norse dragons that have a long, snake-like body and no legs. Wyverns are commonly seen in heraldic devices, and have only two legs. Hydras are many-headed dragons, and Eastern dragons are generally the good kind.
The Whole Bushel
Dragons are a hugely diverse group of mythological creatures. We know them when we see them, even though they can look very different. What we might not realize is that there are names for all of them.
According to the ancient Greeks, the original dragons were a serpent-like creature with a heavy head; in some cases, they had more than one head. The very word for dragon is derived from the Greek words drakein and derkomai, meaning “to see clearly.” It’s the Greeks that also first give us the idea of a greedy dragon guarding its hoard. The Hesperian Dracon, the Colchian Dracon, and the Ismenian Dracon each guarded treasures in the ancient world. Greek dragons were also noble creatures, though, and some served alongside the goddess Demeter and the witch Medea.
If “dragon” is the genus, there’s huge variety in the species of dragons.
The hydra is a many-headed dragon. The most famous one is perhaps the one that gave the group its name, the Hydra, a nine-headed dragon that would regenerate one head for each one that was removed and was eventually killed by Heracles.
A wyrm (or worm, or vurm), is a type of dragon that commonly appears in British and Norse mythology. These dragons have no legs or arms, and only the long, scaled body of a serpent. Often, they are often given poisonous breath and the ability to regenerate or reattach pieces of their body that have been cut off.
The wyvern is a type of dragon that’s often seen in heraldry and as a part of family crests. These dragons have only two legs and a long, serpentine tail that they can sometimes use to support their bodies. They are also usually—but not always—depicted as having wings.
Sea monsters are also considered a type of dragon. These obviously water-dwelling creatures share many of the same physical and psychological characteristics of their land cousins.
According to the Greeks, the classic, reptilian dragons evolved from a different type of dragon. Half woman and half reptile, the water goddess Ceto gave birth to others like her, such as Echidna. Echidna, in turn, gave birth to some of the first “real” dragons of mythology.
Interestingly, it’s thought that the relatively modern idea of the dragon was developed from perhaps the least typically dragon-like of all the Greek dragons, the Chimera. With its long, serpentine tail, it was considered one of the different types of dragons, but we rarely think of something that’s part lion and part goat as being very draconic. Later, medieval paintings of Saint George and his famous dragon have been based on earlier depictions of the Chimera in Greek art.
The typical mental picture that most people have of a dragon, with its four legs, wings, and ability to breathe fire was actually a much later construction than dragons like the wyvern and the wyrm, and is thought to have been developed by the Romans based on these earlier creatures.
The serpentine dragons of Eastern mythologies are very, very different from their Western counterparts. These dragons typically figure into the creation and continuation of the world as we know it, and are more often benevolent creatures that control water and are commonly linked with the powers of light, heaven, and the yang.
Show Me The Proof
Dragons of Ancient Greek Mythology
Fictitious and Symbolic Creatures in Art: Chimerical Creatures of the Dragon and Serpent Kind: The Wyvern
Mysterious Britain & Ireland: The Dragon in Folklore & Legend
American Museum of Natural History: Asian Dragons