Draco nobilis,the Great Dragon, is the most familiar of Pratchett's Dragons.
Similar in form to the European dragon, the Noble Dragon remained mysterious. Eccentric enthusiast Lady Sybil Ramkin pointed out in Guards! Guards! that they used to appear from somewhere (With a lower gravity?) but they have stopped, a view shared by official Tyrant Lord Vetenari in spite of those mysterious incineration-related disappearances of parts of Ankh-Morpork. The Dragon had been summoned by The Unique and Supreme Lodge of the Elucidated Brethren of the Ebon Night, who found the trouble with controlling the hard and hot way.
Rincewind said in The Colour of Magic that Draco nobilis may not exist for most of the time. To quote: "The true dragon is a creature of such refinement of spirit that they can only take on form in this world if they are concieved of the most skilled imagination. And even then the said imagination must be in some place heavily impregnated with magic,which helps to weaken the walls between the world of the seen and unseen, then the dragons pop through, as it were, and impress their form upon this world's possibility matrix.".
However, when he landed on the Disc's moon in The Last Hero he found silver dragons, feeding on the silver moon and with no fear of humans. However, no more spacecraft may be built (By divine order.) and the unknown dragons are left to sail on through the void.