From Discworld & Terry Pratchett Wiki
"Draco vulgaris" literally means "common dragon". These dragons are generally called swamp dragons because they evolved in swamps, where there is rather little that can be used as fuel, and this is a problem for animals that create flame to incubate eggs, fight off enemies, predators, and other dragons (competitors for food or territory), or just to dispel boredom. Swamp dragons compensate for this by evolving a huge appetite for anything that can be used for combustion. Swamp dragons can rearrange their "internal plumbing", guts, stomach, other miscellaneous tubes, to make the best use of what they have eaten, and to make the hottest flame they can. When having indigestion (a common ailment for swamp dragons), or being over-excited, a dragon tends to explode, which is the most common (practically the only) cause of death for swamp dragons. Swamp dragons are almost permanently ill; the famous swamp dragon breeder, Lady Sybil Ramkin, has written a book listing all swamp dragon diseases, their symptoms, causes, treatments, and so on.
Swamp dragons can grow up to about two feet long. All have wings although for some the wings are only ornamental. Swamp dragons can fly, and have to answer to real physics when they do (see Noble dragon (Draco nobilis) for a comparison).
Nowadays, there is a trend among nobles and other rich people to keep swamp dragons as pets. There are breeders' associations, many breeders being rich enthusiasts rather than commercial mass-producers. There are competitions and judging. There are many breeds of swamp dragons with different looks, different personalities and, last but not least, different propensity toward exploding. Swamp dragon breeders are careful to give their dragons the right amounts of coal, charcoal, and some other supplements. This does not always keep the dragon in one piece.
There are always owners who throw out their swamp dragons, just as happens with any other pet species. In Ankh-Morpork, Lady Sybil aids Rosie Devant-Molei's Sunshine Sanctuary for Sick Dragons, which houses abandoned swamp dragons rather than being a vet hospital for swamp dragons. It is unclear whether such charity is practiced in other cities of the Sto Plains. Another notable dragon breeder is Brenda Rodley, Her Grace the Dowager Duchess of Quirm.
Swamp dragons are described in detail in Guards! Guards! and Men at Arms, in which the dragons have rather important roles. A drawing of pet varieties of swamp dragons, with a brief list of their characteristics, can be found in The Last Hero. Interestingly, this (non exhaustive) catalogue of dragon varieties lists two named for Sybil Ramkin, so she has evidently had some success as a breeder of pedigree dragons.
Swamp dragons may also make brief appearances alongside Lady Sybil, in books of the Watch Series. There is, for instance, Dribble, an elderly house dragon who keeps vigil under the cot belonging to Young Sam Vimes, and who is the third person present at six o'clock every evening for the nightly performance of "Where's My Cow?"
When the Ramkin house is invaded by a Dwarf assassination squad with flamethrowers, the dragons in the pen see this as offence. This leads them in attacking one of them with their flames, and the result is a pair of iron boots melting into the floor. And Sybil hands her husband Raja, a Golden Wowser (a breed with an intensely hot directed flame, often used as the equivalent of an oxyacetalyne welding torch) as a retaliatory weapon..
The collective noun for swamp dragons is "a slump" or "an embarrassment". An interesting clue as to why swamp dragons are as disastrously ill-equipped for life on the Disc as they seem to be has been gleaned from the recent (re)discovery of moon dragons.
More of page image Poco the Dragon-Lizard may be found [here| http://1977thecomic.com/], copyright W. Byron Wilkins.