The Dragon was the name given to Lews Therin Telamon, a powerful male channeler in the Age of Legends. After going insane from the taint on saidin, he took part in the Breaking of The World and killed his own lover, Ilyena Sunhair. When he died, he created the mountain of Dragonmount.
The Dragon is usually thought of as evil, sometimes conflated with the Dark One. He is widely known to have triggered the Breaking of The World. There are some who quietly believe, perhaps because of the Prophecies of The Dragon, that he will return and save the world in its time of greatest need.
A dragon is also a mythical creature, unknown in the Third Age.