Inevitably when someone says dungeons, you think about dragons. For so long it has been the foremost alliteration that has been used almost synonymous with our hobby. But why?
There's still some speculation where dragons actually came from. But it is one of those things, that no matter the culture you grew up in, you will have heard about them. They of course have been featured in our favourite works from Tolkien and the like, but their stories go back to things like Beowulf or norse mythology.
Central to the plot is always, that dragons are these powerful beings that can only be defeated by the fearless knight. We wanted to be that hero in the story, so we built a game around it.
Dragons do not need to be this literal though. It is more of a metaphor. It is a powerful being that sets itself apart from the other monsters, like goblins or orks. It has motivation and purpose.

We have seen these two words before when talking about dungeons, and they fit here as well. Many other monsters might have a motivation sure, but a dragon develops a plan from it. It uses cunning and intelligence. They use their power to enact that plan. And in the end, that plan will fulfil a purpose.
When these plans are in conflict with the values of the heroes a villain is born. The BBEG. The dragon. It isn't because they are necessarily evil, it is just that they oppose current morale and ethics. The best villains, in my opinion, are those that make us think, if maybe they are right, just the way they go about it is wrong.
Thanos from the MCU is a great example for that. His motivation is to safe the universe. The plan is to halve its population so that there's enough resources for the survivors. The motivation is benevolent, the plan not so much. So he comes into conflict with the rest of the universe, as he goes around killing people. In the end he gets slain by the fearless knight: Ironman.
The important thing is, that the dragon or BBEG is acting with intelligence. They are using their power like a surgical tool and only when necessary. The heroes will first not directly come in contact with them, but with the effects of the plan. After all, the heroes were never part of the initial plans, they were never part of the story. It's by coming into contact with these plans, that the conflict is starting and the story unfolds.
Vaesen has the notion of the Countdown. A series of events that happen over the progress of the adventure. Each progress also provides more insight onto the motivation of the monster and as the investigators to their thing, the countdown changes.
Similarly a BBEG should behave. There's a predetermined set of steps in their plan. If the heroes do nothing, these will lead to success and, probably, the end of the world (or something like that). But every step leaves ripples, that can be felt.
This way you end up in a believable and living world. A world with dragons.