The Primal Dragons

Many myths abound that explain the Dragons. Why there is a dualistic quality to their quantity. Five who have a hue that matches the rainbow, and five who possess the sheen of precious and valuable metals. Five benevolent and five malevolent dragons to traverse and terrorize the world.
It is said that in the golden age, two dragons were felled, one crimson, the other gold. At the time it was thought that that was the end of those dragons, but in time it became apparent that they would reincarnate in a new form.
Since then, the primal dragons have hidden themselves from the world in places were people have difficulty finding them.
Dragon spawn such as the Dragonborn, Winged Drakes and Guard Drakes all descend from the original ten, although only Dragonborn are sapient.

Historical Basis

While there are Several different forms of dragons, empirical data proves that they are all descended in one form or another from one of the Primal Ten. Captain Salazar (The human persona of the primal Bronze Dragon) volunteered this information to famed explorer Lord Therin Greystone.
"It's several things actually. I'm not entirely sure of the rules myself, but I do know this: I mate with a humanoid in my humanoid form, they give birth to a dragon-born. I've heard of the chromatic ones mating with wyverns to produce winged drakes, though that seems abominable to me. If I mated with silver, we'd hatch a wingless clutch of drakes. No drake can speak though they are intelligent creatures, they do not have the discipline of sentience."