@overlord has well established that it's reasonably possible for a 500 kg flying animal to exist. If nothing else, your 500 kg dragon could be a long-term soaring creature like an albatross, land only on prominences (cliffs and peaks) by preference, and avoid the need for a level ground takeoff almost all the time.
He needs to be able to take off from flat ground, though, because landing on the prairie or beach being a death sentence makes it hard to support a species that pretty well have to be carnivores (or at least carrion eaters).
There are sequence drawings, perhaps even video, of an ergonomically possible way Queztlcoatlus might have been able to launch from flat ground -- in general, it involves a sort of "pole vault" move, striding forward on the wing knuckles, then using the momentum with a stiff-legged "hop" to get enough height to unfold the wings and begin to laboriously flap to a soaring altitude. Obviously, the underlying requirement is that your flier needs to have the strength to climb by flapping, at least at a limited rate (it needn't have the performance of a Cessna, but that of a self-launch sailplane -- 40-50 meters a minute -- is about a minimum).
This climb performance is achieved with as little as 10 kW in sailplanes of similar mass, so let's say our (somewhat less efficient) super-Quetzl will need 15 kw. That only .03 kW/kg, which is pathetic for any living animal. Seems likely to work, given evolutionary pressure for such an animal to grow even larger than the known specimens...