Dragons were ancient, intelligent, powerful creatures, as feared as they were admired in Middle-earth. Their exact origin is debated, though it was clearly stated that they were created by Morgoth in some sense, millennia ago.
Dragons lived throughout the First, Second, and Third ages of Middle-earth and may have lived longer. They were originally bred by Morgoth during the first age to serve as powerful war beasts. The first dragon ever seen in Middle-earth was Glaurung, the father of dragons, whom Morgoth used to great effect during the fourth and fifth battles in the War of the Jewels. Like most of the creatures made or twisted by Morgoth, they were capable of reproducing independently. Dragons were sometimes referred to as serpents, great Worms, or simply Drakes, with the first two specifying the Wingless Dragons.
During the First Age, the greatest heirloom of the house of Hador was the Dragon-helm of Dor-lómin, which was a helm of great weight that had as its crest an image of the head of Glaurung the Dragon.
The dragons of the Second Age and Third Age were a blight upon the Dwarves and all those who hoarded treasure. Some, like Smaug the Golden and Scatha the Worm, went from the Withered Heath and invaded Dwarven kingdoms.
Before the coming of Smaug, there had not been notable activities observed during the Second Age, as they are largely independent while they still posed threats to men along with "demons" and "misshappen beasts", and Sauron gathered "all the evil things of the days of Morgoth that remained on earth and beneath it", possibly including dragons, beginning the Accursed Years.
There was an inn in the Shire called The Green Dragon, and a type of flower and firework called a Snap Dragon. For Bilbo Baggins' 111th birthday, Gandalf made a special dragon firework.
It is presumed that, after the death of Smaug in the Third Age, the great dragons had become extinct. According to Gandalf, the fire-drake race survived until at least just before the War of the Ring, and some lesser kin survived even after the war. It may have been possible that some races of dragon still existed throughout the Fourth Age.