To the OP: The DnD monsters drew heavily on existing legend, and why not! The DnD Coatl is directly lifted from the Mesoamerican deity Quetzalcoatl, also called the Feathered Serpent.
Of course, Quetzalcoatl could have been inspired by a snake-eating bird. Particularly in S America, where 3m Tall avian monsters with hooked bills as long as your arm used to be the apex predators ( The Phorusrhacidae ).
Never turn your back on a chicken...
