Sigh. I really thought that after everyone thought that the flying Muto was Rodan before that debunked, we'd be done with stupid crap like this. The shadow is of a jet falling from the air as the Muto approaches. Why does it look like it has unique movement? Because fog is not a flat projector screen, it's a three dimensional shape. The shadow is rolling off the fog in an area that is not flat, giving it the illusion of more organic movement. Adding to the effect is that the jet is spiralling as it falls, which is an unnatural type of motion for a jet. Combine that with the fog and you have something that looks like more than what it is.
I know you want to think that there's an easter egg there, but there 100% is *NOT*. I'm not convinced there are any Ghidorah easter eggs, but assuming there are the only things that make sense is the Chinatown dragon sculpture and possibly the origami cranes (three of them flying, could be a reference to the Dorats). Also, Edwards and Legendary did NOT have the rights to Ghidorah, or any other kaiju than Godzilla from Toho.
I get that you really want to believe this is some easter egg that no one caught that retroactively proves Ghidorah would make an appearance, but it simply is not. The reason no one "caught" it before is because there's nothing to catch. I doubt I'm going to convince you because of how irrationally hard you are clinging to this idea, but seriously dude, you are projecting something that makes no sense into a minor detail that wasn't supposed to add anything to movie besides a touch of realism and drama.
