Look more closely at the shape and construction of the wing of the airplane; that's a modern craft gentlemen.

i'd say its probably the plane's shadow and only looks to be flapping due to the distortion against objects on the ground

One time when I was walking about, I looked up and saw what looked like a manta ray, it was about as high, if not higher than most planes fly in this area, I even had a look at it with binoculars (Which are among the things I carry in my backpack on a daily basis), it was very strange.

That was some of the craziest stuff ive ever seen.

Its interesting to say the least, my guess is fake, but then again... flying manta rays? it sounds strange but to a degree i could believe in the possibility, but at 20,000+ ft? 300+mph? doubtful.

Great OP!! Im saving that footage

perhaps it's the fabled mothman? not that I find this video particularly convincing unfortunately.