So, first off I'm a bit of a nerd and one of my favourite things about the paranormal is trying to explain it but if that's not ok here I'll shut up and,
Second, if this isn't in reference to me, ignore this, but:
The Quetzalcoatlus was a large but thin pterosaur that mostly glided like birds such as condors and vultures and I am not saying something as large as Mothman couldn't fly with a normal metabolism but that something as large as Mothman wouldn't be able to fly at the speeds and turn with the agility it can.
also not necessary to my argument but I do feel like I should mention that in the late Cretaceous the oxygen content in the atmosphere (oxygen being the primary energy source in metabolism) was 30% in some projections compared to the 20% we have today so it would have been able to gain more energy from breathing.