Now my uber-geekness comes about:
Dragons don't have to have four legs AND wings. St. George's Dragon has several images of only 2 legs and 2 wings. Chinese dragons are said to have only 4 legs and no wings.
Also, Dragons are more probable given the fact that there ARE giant lizards with nearly acidic breath (one bite and you'll lose an arm, and likely your life) These were even named Dragons.
A wyvern would be a feathered version of St. George's Dragon. Quetzalcoatl would be a feathered snake.
A two-legged dragon having wings and a barbed tail. The big point being two legs...