It has been theorised that century old reports of feathered flying serpents in Wales may have been some sort of unusual bird, perhaps a pheasant. It makes sense, unlikely as it seems, the Quetzal bird was worshiped as a flying serpent by the Mayans, because it appears serpentine in flight. Like the Quetzal and the peacock, it is said to glitter and shine, with eyed wings and tail appearing like jewels. The birds were reported to raid chicken coops, either this is scapegoating, and the culprit was a fox. Alternately, some large birds, including pheasants, are very aggressive towards chickens (also, large birds invariably attack each other), and many kinds of domestic pheasant are cannibalistic.
Thanks to Karl Shuker for introducing me to this Cryptid.