Actually, the "dragon" which Lord Soth confronted in Strahd's castle may not have been the genuine article. In the original I6 module, four dragons *statues* in Castle Ravenloft could come to life as (small) red dragons to attack intruders. Given the rarity of such beasts, Strahd's replacable "dragons" might make more sense as golems or oversized Figurines Of Wondrous Power than as true dragons; in the "House Of Strahd" 2E module update, they were replaced with gargoyle golems, IIRC.
The "dragon" that Elena Faith-hold claims to protect her people against is, in fact, Elena herself. She warns Nidalans that she's the only thing which prevents the (nonexistent) dragon from devastating their lands as part of her propaganda ... then razes villages which fail to live up to her fanatical standards of righteous behavior, while blaming their destruction on "the dragon". [Of course, IYC you might rule that the Dark Powers have since planted a REAL dragon in her domain, to make her life miserable....
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In general, for dragons to play a role in a Ravenloft campaign, it's best to ignore all the conventional D&D ideas about dragons (like their social structure, their pantheon, etc), and revert to the classic Euro-archetype of the dragon as a symbolic embodiment of greed (Fafnir; Smaug) and evil (St. George's dragon; the Beast of Revelations). Play a Ravenloft dragon like you'd play a fiend, without any redeeming qualities to humanize it, and it'll fit into its own thematic niche, just as the more traditional Gothic monsters do (e.g. vampire as seducer, mummy as implacable avenger, etc).
I remember one of the examples from the 2E "Complete Book of Villains", in which an evil dragon had established a cult of greed to serve it. It taught its followers to take, take, take, and to demonstrate their commitment to avarice by bringing the most generous, altruistic person they could find, and delivering them to the dragon to be devoured. In the end, one of the cultists became so greedy that he held back a single coin from the tribute the dragon demanded of its followers; spotting the discrepancy immediately, the dragon chastized the *other* cultists for failing to learn what it had taught them, praised the cheater for being an excellent student, then burned them all to death (and kept their treasure, of course).
*That* dragon, if it went on to found another such cult in another land, to bask in their gifts and chow down on their kidnapped offerings, and then to slaughter them all the moment one of their number worked up the nerve to cheat it -- and then another cult, and another, and... -- might just find itself right at home in Ravenloft. But a dragon that acts like just another NPC ... no way.
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