Nightmare Moon wing headcanon!  
So I Decide to give NM the feathered bat wing mostly b/c of how her wings look in the show. There is almost an “arm” looking appendage along the top of her wings, they are pointed and it looks like there are 2 layers of feathers at the base of the arm.  
NM- Nightmare Moon’s wings are the second largest of the alicorns with Celestias being the largest and Lunas being the 3rd largest. She has batwings that are internally laced with 2 layers of feathers. Unlike other batponies NM has 4 “fingers” rather than 3, this provides more control over her ability to curve and stretch her wing tips. The layers of feathers inside her wings are for both warmth and sound reduction although her flight is not silent. The skin behind the feathers is semitransparent in the same way her mane is. The stars on her feathers are accurate with the ones in the sky, in the daytime they don’t show up but as earth rotates at night the the stars on her feathers move identically with the ones in the sky.  
These are fun, I’ll do more soon. I have a good theory for why Scootaloo cant fly ^_