Krayt dragons are massive reptiles that live in Tatooine's Dune Sea. They are extremely rare, and are only encountered on a few occasions, and by very few individuals. Their rarity is due in part by the fact that they have been hunted for millennia for their pearls, which are of extraordinary value.
Revan encountered a Krayt dragon on Tatooine in his search for the Star Forge. A Twi'lek hunter was waiting outside the lair, deliberating on the best way to tempt the dragon out when the former Dark Lord came along. The Twi'lek told Revan of his plan on drawing the dragon out into the open, where he had mines set up. Revan brought several Banthas to tempt the dragon, who left its lair and walked through the mine field, which killed it after several blasts.
Krayt dragons are also hunted on Tatooine in Star Wars: Galaxies.