As time moves on, so does the game Dungeons and Dragons. Over the years there have been 5 different editions of the game, each with their own rules and creatures. Inevitably some creatures were forgotten along the way, buried in supplements, or simply left to be referenced once or twice in D&D literature.
There have been so many types of dragons created over the years it may well be difficult to catalog them all, however, these are some of the rarer ones. Some of these have a stat block in 5e currently and some don't, either way, each of these dragons has their own unique traits and history. A creative DM might even find one of these dragons better suited to an adventure they were planning than any dragon in the current 5e Monster Manual.
10 Mercury Dragons
These dragons are a moody lot and are quick to swing between emotions. They have a history of being a type of dragon which varies in personality depending on their personal experiences. Mercury dragons in hostile environments grow up to attack others on-site, but on the other hand are gregarious and friendly if raised in an environment that rewarded or reinforced such behavior.
They crave new experiences and as such seek out the different and odd to test and prod it. They are known to take on humanoid forms quite frequently in their search for new experiences. Oddly, their breath attack is a searing ray of light that burns everything it touches.
9 Iron Dragons
Reputedly the most powerful of the ferrous dragon types, it oddly dislikes combat and will avoid conflict where it can. A neutral dragon in alignment, it seeks out knowledge about the world around it. Willing to travel great distances in the guise of a humanoid to achieve this, the Iron dragon is not as naturally hostile as some dragon types may be.
Their breath attack is a bolt of lightning. They once lived in mountainous regions but since the spell plague (a cataclysmic event) they shifted their natural habitat. They now mostly reside in forests and hilly regions where they can hide their bulk and ambush prey. This shift in habitat is odd though as it brings them into conflict with green dragons frequently.
8 Steel Dragons
Steel dragons are a very odd type of dragon. Their dragons hoard isn't necessarily treasure, as they have a tendency to spend a portion of their life producing a "work" which makes up the bulk of their hoard. They live hundreds of years but live almost exclusively as a humanoid creature for most of this time. Taking on different "lives" which they live all the way through till they "die."
Upon their "death," they turn back into a dragon and meditate on what they have produced or learned. What they have gained from this life they add to their hoard, this is their work which they build on. They will then transform into another humanoid and live that life, and will repeat the process until they reach their natural life span as a dragon. As a result, no one can really know where one of these consummate method actors are or their preferred habitat.
7 Platinum Dragons
The rarest of the metallic dragon types, the platinum dragon is believed to also be the most powerful. The species is so rare that scholars only know of one in existence, Bahamut. Bahamut is revered by gold dragons as a god, and whether he is one or not, it can be said if any other platinum dragons exist they are likely as powerful.
In early editions of Dungeons & Dragons, the platinum dragons were one of the few dragons that did not have wings and were serpentine. Though Bahamut has always had wings, all the dragons of this type though potentially looking different from each other, are all god-like in their power.
6 Brown Dragons
Known as desert dragons, the brown dragons hide beneath the sand. They prefer the solitude of the wild far away from settled areas so that humanoids will not bother them. They can swim through sand like water as well as take to the air in flight. What makes them unique among dragon kind though is their love of food.
The brown dragon has a desire for diverse spices, textures, and tastes. Very much a gourmand, the dragon will go out of its way to get any unique food it can. This also extends to its treasure hoard, which is replete with only the most expensive cutlery and sets of utensils.
5 Purple Dragons
These dragons live beneath the surface in the Underdark, far deeper down than any other dragons tend to go. They have two loves in their life, one is exploration, they live to uncover and delve deeper into the world below spurning the surface world as uninteresting. Their second, and more malicious interest, is the pursuit of dominating others.
The purple dragon seeks to control others and manipulate them through domination, trickery, or lies. This may be similar to a green dragon's interests, but the purple dragon differs. It does not seek to dominate others to further its schemes but simply because it can, and no other reason is needed.
4 Gray Dragons
Also known as fang dragons, the gray dragon does not have a breath attack (for the most part) as other true dragons do, but it does have one of the most ferocious bites amongst all the dragons. In some cases, it's even said that its bite is similar to that of vampires as it drains a portion of their foe's life essence. Its claws as well are reputed to be more dangerous than that of the average dragon.
Enigmatic hunters and killers these dragons don't have a favored type of treasure, instead, they collect mementos of their prey. These creatures for all their size and ferocity are opportunity killers, not looking for fair fights but the thrill of the hunt.
3 Yellow Dragons
With a long and serpentine body, the yellow dragon does not get along with the rest of its chromatic dragon kin, although it is evil (or sometimes neutral) in nature. Tiamat, the chromatic dragon god does not claim this as her own creation and does not have a yellow head on her body.
The yellow dragon, like other rarer chromatic types, are hunters by nature. Planning carefully thought out and meticulous traps, they ambush their prey with a watery breath weapon that has corrosive compounds in it. Their scales are a dull shade of yellow to not reveal their position when they hunt in the desert environments which they call home.
2 Concordant Dragons
These are astral dragons tasked with maintaining the balance between the good and bad dragons throughout the planes. They are unique given their breath weapon damages individuals based on the moral alignment, the further from neutral they are the more damage they take. Little is known about this type of dragon, but a well known concordant dragon is Asgorath.
Asgorath was a dragon god of creation and a greater deity, this gives some insight into just how powerful these dragons can be. They are also one of the few true neutral dragons that exist in D&D and live on the outer planes.
1 Hoarder Dragons
Dragons all collect a hoard of wealth over their life, but if a dragon becomes too greedy and too obsessed with their wealth it can cause the dragon to live on after death. The dragon's spirit physically becomes a part of its hoard. The great blades in its collection becomes its teeth and claws, shields and gold plates becoming scales, and all the rest of it is made from its trove of coins.
This dragon will attack any being that it believes covets its wealth. Its wealth, which has now become its body must be defended and added to at all times, making this ravenous dragon spirit extremely dangerous.