Metallic Dragon
Metallic Dragons are one of the many specific breeds of dragon native to Dungeons & Dragons and Pathfinder. They are one of the two most iconic family groupings of D&D dragons, alongside the Chromatic Dragons. Lesser D&D dragon families have include the Catastrophic Dragons, Gem Dragons, Oriental Dragons (or "Imperial Dragons", for Pathfinder) and Planar Dragons.
In an expansion of the alignment mechanic, chromatics helped define what our frenemies on TVTropes calls "Color Coded For Your Convenience", where the general color-scheme of a dragon immediately helps you figure out what they are as a general rule.
Metallic Dragons are based on the colors of various metals and are traditionally portrayed as the "goodly dragons". There has been some contention about this over the years; as most metallics are "noble" metals like copper, gold and silver, "baser" metal-based dragons were given a tentative life as the Ferrous Dragon sub-family in Dragon Magazine#170 and #356, for AD&D and 3rd edition respectively.
The patron god of all Metallic Dragons is Bahamut, whose status as being based on platinum makes him unique.
Bronze Dragon
Brass Dragon
Copper Dragon
The Copper Dragons are most known for being the practical jokers of the Metallic Dragons.
Gold Dragon
Regal and imperious, Gold Dragons traditionally hold the position of being the mightiest of the Metallic Dragons, although that has been shaken up before - most notably in 4th edition. Originally, Gold Dragons were designed to more of an "eastern dragon" body-structure, with long, serpentine, wingless bodies and flowing mustaches. Their bodies have changed to match the standard "western dragon" archetype, but the pseudo-mustaches seem here to stay.
Silver Dragon
Although inferior to Gold Dragons on the power scale, Silver Dragons are seen by many as being the "Most Good" of the Metallics. Having the personalities of knights in shining armor, Silver Dragons actively crusade to stop evil, whereas Gold Dragons prefer to focus on more abstract, philosophy-based approaches.
Adamantine Dragon
In 4th edition, WoTC decided that "why the heck should Bronze and Brass be core members of the Metallics? They're freaking copper alloys, not proper metals!" As such, they bumped them out of the core quintet and relegated them to the Metallic Draconomicon. That left two niches to fill to make Metallics remain on equal grounds with their Chromatic enemies, and so they filled them in. Adamantine Dragons are heavily-armored Metallics native to the Underdark, who may arguably be more powerful than even Gold Dragons, with distinctive beak-like snouts and a breath weapon that deals thunder damage. These dragons were entirely unique to 4e; AD&D had featured an Adamantite Dragon, but that was a Planar Dragon native to the Twin Paradises of Bytopia.
Iron Dragon
In 4th edition, WoTC decided that "why the heck should Bronze and Brass be core members of the Metallics? They're freaking copper alloys, not proper metals!" As such, they bumped them out of the core quintet and relegated them to the Metallic Draconomicon. That left two niches to fill to make Metallics remain on equal grounds with their Chromatic enemies, and so they filled them in. Iron Dragons originated in Dragon Magazine #170 as members of the Ferrous Dragon family - see that page to learn about how they looked there. As a core member of the Metallics, 4e's Iron Dragons are essentially a Metallic analogue to the White Dragon, being simple-minded, brutish and violently aggressive dragons who rely on electromagnetic attacks and thick scales to fight their foes.
Cobalt Dragon
Like the Iron Dragon, Cobalt Dragons are a Ferrous Dragon breed that got promoted to Metallic in 4th edition, with quite a lot of differences. The 4e Cobalt Dragon is a grim, vicious, possessive, powerfully built Metallic Dragon that looks a lot like a flying bear. They absolutely hate the heat, establishing tyrannical dominions in colder regions where they are comfortable. Possessed of an exorbitant amount of martial pride, Cobalt Dragons prefer valuable arms and armor for their hordes, and relish commanding skilled, war-like minions.
Mercury Dragon
Fickle, whimsical and highly chaotic, the Mercury Dragon is an obscure beastie that is perhaps native to the Forgotten Realms, having first appeared in the AD&D Monstrous Manual, followed by 3rd edition's "Dragons of Faerun" and then the 4e Metallic Draconomicon. Traditional Mercury Dragons use laser beam breath weapons and reflection-based attacks. The 4e version is a superb shapeshifter that might as well be made of living metal - picture a draconic T-1000 Terminator - and wields a poisonous breath weapon.
Mithral Dragon
Unique to 4th edition, Mithral Dragons are the elite of the Metallic Dragon family tree, native to the Astral Sea and wielding a diverse array of powers that allow them to bend space and time to their will.
Orium Dragon
Entirely unique to 4th edition, Orium Dragons are red-gold colored dragons, with serpentine heads, necks and tails mounted on a rather feline body frame. Based on "orium", WoTC's trademarkable name for "Orichalcum", they are obsessive historians who seek out ancient ruins to maintain, preserve and restore. Their unique breath weapon is a gout of toxic vapor, which then coalesces into a snake-like construct that keeps on fighting on its own after being exhaled.
Steel Dragon
The earliest form of the Steel Dragon was known only as the "Greyhawk Dragon" or the "Oerthian Dragon", but was later reprinted with new artwork and a new name as a Steel Dragon. It's not entirely clear if Steel Dragons are recognized as Metallic Dragons or not prior to 4th edition, but their characterization has remained remarkably consistent. Steel Dragons are fascinated by humanoids and humanoid culture, using their shapechanging ability to live amongst humanoids and enjoy being part of their communities. 4th edition added a certain chaotic tinge to the race; the 4e Steel Dragon is a huge believer in personal liberty, and cannot abide tyranny - to the 4e Steel Dragon, law exists to serve the people, not the other way around. This makes them surprisingly bitter enemies of some breeds of Metallics; the readiness with which Bronze and Gold Dragons will take absolute authority and justify it as "I know best" incenses the 4e version of Steel Dragons, who do their best to take these "benevolent tyrants" down a few pegs.