Mankind has many myths about dragons throughout the world. In the Orient the dragon is featured on pottery and in parades, and European myths are replete with Knights in armor going into battle against firebreathing dragons. The dragons invariable have wings and are shaped suspiciously like dinosaurs. Is there some basis in fact for this common human memory? Indeed there is. Where the dinosaurs died out long before mankind was engineered into being from ape stock, the Earth was not the only planet to give birth to such creatures. Evolution follows similar patterns, and where your Earth brought forth dinosaurs with wings, just so did other planets compatible with the Earth.
Your dragons were transplants brought to the Earth by early ambassadors from the Service-to-Self, who anticipated an increased harvest of lackeys from the Earth due to the fear and despair their dragons wrought. Were the dragons intelligent and incarnated with souls? No. On a scale of 1 to 10, where the average dog is a 10, a dragon was barely a 3. This is why, in spite of the massive size and capacity to fly as well as lumber over hill and dale, they were so easily slain. Dumb brutes. Hundreds were brought to Earth, and released on terrified villages. The dragons ate humans, which were neither swift runners or able to barricade themselves in their mud huts, and were a tastier morsel than the deer, which ran fast and had taut, lean muscles. The legends speak of sacrifices, where humans would periodically offer one of their own to placate the dragon. Well, of course this reduced the rampages, as dinner had been brought to the door.
Where some dragons mated, they were scattered to begin with and, dying without progeny, eventually disappeared from the Earth. There were perhaps a few hundred delivered, and their reign, ending only in the last Millennia, lasted for thousands of years. The stories of dragons, all the more impressive as they were true, spread more widely than the dragons themselves. Did they breath fire? No. But when roaring, open mouthed, it felt like that to those within range. Hot breath and a big wind pipe.