There are different classes of dragon, but the main distinction resides in whether of not they can fly. Within each of these two species, we encounter fire dragons, capable of projecting enormous flames from their mouths, and dragons of the cold, with icy breath which freezes their victims. The origin of these terrible creatures can be found in the epic The Silmarillion, published after the death of Tolkien, which recounts the whole history of his imaginary Middle Earth. In this work we read about beings created in the First Age by Morgoth. The first to appear in the chronicles was Glaurung, a fire dragon who could not fly. Glaurung took part in the Battle of the Sudden Flame and in the Battle of the Unnumbered Tears and was finally killed by the hero Túrin.

However, the most terrible of thse creatures was Ancalagon the Black, an immense winged fire dragon, who died in the War of Wrath, felled by a well-aimed arrow shot from Eärendil, while he was fighting against the great eagles. It is said that when he fell, the impact of his gigantic body against the mountains of Beleriand caused a cataclysm which changed the geography of Middle Earth.

After the War of Wrath, during the second and third Ages, the dragons took refuge in the remote north, in the region known as the Dead Marshes, or in caves in the Misty Mountains. From there, from time to time, they descended south and brought chaos and destruction in their wake.

Of all those who remained in Middle Earth, the greatest and the most famous was Smaug, who was also a winged fire dragon. At the end of the Third Age, he devastated the kingdom of the dwarves in Erebor, the lonely mountain, and seized the vast treasure. How and why is told in The Hobbit, where the dwarf Thorin Oakenshield organizes an expedition to recover the treasure storlen by Smaug and to avenge the destruction the dragon wreaked on the dwarves. He fulfills his aim when the Hobbit, Bilbo Baggins, manages to trick the dragon and helps Bardo, a human hero, to kill him.