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Malbeth the Seer
Dragons, the archetypical monster and supernatural creature of most mythologies of mankind, also have their place in Tolkien's Middle-Earth.
Often also named drakes (Old English), worms or wyrms there, dragons in Middle-Earth, are, however, exclusively evil creatures; created or bred by Morgoth, the primeval Dark Lord, in the pits of Angband in the First Age. What their true nature is, and what spirits inhibit them, is still a point of uncertainty, and much debate (as a look on the forum topics on dragons quickly reveals).
All dragons were huge and fierce, and possessed from the beginning a great evil cunning, and they could speak with enchanting voices. The first of them that was released upon the unsuspecting Elves in the Dagor Bragollach, appropriately called the Battle of Sudden Flame, was Glaurung the Golden, father of the Dragons: he was clad in thick scales, and might be imagined to have looked like a huge long lizard on four mighty legs, laying lands to waste with his fiery breath. Later, in the War of Wrath, Morgoth let loose the first flying dragons upon his enemies, of which Ancalagon the Black was the greatest.
The few dragons who survived the War of Wrath and the downfall of Morgoth and his stronghold, removed to scarcely inhabitated regions in the north and north-east of Middle-Earth. The Withering Heath in the eastern Grey Mountains was from then on ever a dangerous region as great dragons still prowled there.
However, with Morgoth gone, they were no longer under anyone's control, as Sauron never mustered the strength to command those mightiest of Morgoth's creations.
Therefore, the dragons used to take abodes of their own, devastating dwellings of the free peoples, and gathering about them great hoards of riches; Scatha, who was slain by Fram, a Northman, and Smaug, the dragon in The Hobbit, both possessed great treasures robbed from the dwarves.
Most we know about dragons is handed down in the stories of a few individual specimen and their slayers.
Glaurung the great Worm of Angband was inevitably and fatally tied to the doom of Túrin Turambar; in a way it was him that carried out the curse of Morgoth. At the Dagor Bragollach, he was not come to full growth yet, and could be warded off by the doughty dwarves. Later, however, when Túrin was staying in Nargothrond, Glaurung devastated it, by then already virtually the captain of Morgoth's troops, and the enchanting glare of his maliciously intelligent eyes created the dreadful fate of Túrin to abandon Finduilas and marry his own sister. When he went to seek Túrin again, Glaurung was slain by him upon crossing a gorge, being stabbed by Túrin's sword into his unprotected belly (a characteristic of most dragons, it seems); but not without revealing Túrin's many errors before passing away.
Ancalagon the Black was said to have been the greatest of the winged dragons, and was released unto the foes of Angband in the War of Wrath. Eärendil, bearing the Silmaril, and accompanied by the great eagles, slew him in his airborne ship.
Smaug was the dragon who expelled the dwarves, among them Thorin Oakenshield, from their kingdom under the Lonely Mountain Erebor.In the great adventure of Bilbo Baggins, Gandalf and the party of dwarves under Thorin, he proved to be a cunning foe, and terrible enemy, as he, roused by the dwarves, descended unto the nearby Laketown, incinerating it, but in turn being slain by Bard the Bowman, who found the only weak spot on the dragon's jewel-protected belly.
The battle prowess, supernatural abilities, and malicious intelligence made dragons most formidable opponents, against whom the group tactics applied by Elves and dwarves availed little except keeping them at bay for a while. In order to slay a dragon in Middle-Earth, it needed a wretched hero, bound to and chosen by fate, armed with a legacy, magic weaponry and not seldom a fey spirit, who would face the beast alone, often meeting his own doom in the encounter.
(See also: Erebor, Glaurung, Morgoth, Smaug, Túrin)
(References: Silmarillion, chapters XIII, XVIII, XXI, XXIV; LotR, app. A, B; Hobbit, I, XII-XIV; HoME 10, II)