One theory proposes that our primate ancestors needed a way to evoke three of our most dangerous predators (leopard, python, eagle) once we decided to walk on the ground. It's called the "snake/raptor/cat complex":
Faced with information overload, the brain of Australopithecus resorted to lumping information into manageable and memorable chunks. As a result, the cat, the snake, and the raptor were merged into a hybrid creature that had the salient predatory features of each: the face of a feline, the body of a snake, and the talons of a raptor. This is the hybrid “monster” that came to be known as the “dragon.”
The Dish 