We play in the Realms and in two homebrews (plus one third that is discontinued).
In mine, Dragons are rare, but not rarissimo. One may stumble of wyrmlings (especially chromatic wyrmlings) often. From the adult age on, dragons are rare and seldom seen. Those who can are usually polymorphed into a humanoid shape, and their true identity is unknown. Others are happy to deal with wizards and sorcerers only, uncaring about mortal things. And yet others subjugates tribes of humanoids, make them believe they are gods, and use them as a mean to get food and wealth through their pillage.
Metallic dragons raise their own children until they are young, at which stade they are big enough to strive on their own and are encouraged to do so, but chromatic dragons just mate more often and lay eggs anywhere, each apart from the other in a different place, and let the strongest win its right to live. Gem dragons are rare -- about 10% of all dragons are gem. They raise their youngs in a temporary lair, and only until they are very young, afterward they leave them, but often keep an eye on them to protect them if needs be.
Contrarily to standard D&D, blue dragons are nigh unheard off, but one may find brown and yellow dragons competing for territory in the Fire Desert.
The ocean hold dragon-turtle, sea drakes (from Mongoose's Seas of Blood, and maybe those from the FF too), and some other strange draconic beasts.
Otherwise, one may find mock dragons (from Creature Collection). They are thought to be, like the hydras, dragonnes and felldrakes (MM2), degenerate offshoots of true dragons. Mock dragons are prized as big bad pets, and sometimes used as mounts in warfare (a howdah is put on its top, holding archers).
In the other homebrew, the one I play in, Dragons are creatures of legend. They are said to have been the first rulers, those who created the Empire and the Harmonious Order of the civilization (it's an OA setting, about 3/4 Japan-1/4 China). Eventually, they grow tired of managing humans, so they Touched the smartest, most honorables families, that took on draconic traits, and eventually retired totally from human affairs, having left power to this Dragon-Touched aristocracy. Each noble family has its own Mark of the Dragon, a slight difformity, and those who bear it are great sorcerers and the rightful ruler of the country.
Of course, the Mikado is dead without heirs, the Shogun -- who isn't dragontouched -- has assumed rulership, and our Marked PCs, having conspired to take the throne and restore the rightful order of the land, have been exiled to the land of the gaijins...