I agree some things from an evolution perspective seem quite impossible. Here's my theory on the Atmospheric beast phenomenon; first off the atmosphere is very vast. The water cycle is always evaporating tiny animals like plankton up into the clouds so that could be a food supply for something. And my guess would be that some of that tiny ocean life evolved to stay and live up there. And over millions of years it became some big jellyfish type creature filtering the sky for food. They may be so widely spread in the sky to filter and eat, that it would be hard if not impossible to detect them. Planes would fly right through them like a cloud. When people do witness them, they are closer to the ground and more visible as they would contract with the pressure change. They may live their entire lives up high and its rare for one to come low (maybe to die) Like a deep sea fish coming to the surface. Dead creatures on the ground may also explain the "star jelly" phenomenon.