It’s not clear when or where stories of dragons first emerged, but the huge, flying serpents were described at least as early as the age of the ancient Greeks and Sumerians. For much of history dragons were thought of as being like any other mythical animal: sometimes useful and protective, other times harmful and dangerous.
That changed when Christianity spread across the world; dragons took on a decidedly sinister interpretation and came to represent Satan. In medieval times, most people who heard anything about dragons knew them from the Bible, and it’s likely that most Christians at the time believed in the literal existence of dragons.
The belief in dragons was based not just in legend but also in hard evidence, or at least that’s what people thought, long ago. For millennia no one knew what to make of the giant bones that were occasionally unearthed around the globe, and dragons seemed a logical choice for people who had no knowledge of dinosaurs.
Source: Live Science – Are Dragons Real? Facts About Dragons