It’s her sacrum ofc.

@Dracorex
 
It does explain how several times in the show you see a Pegasus use his or her wings almost like a “hand”.

@ED-SKaR
 
The traditional method of naming bones doesn’t really sound much better. It would be called the ‘os magis’.
 
@TheAntiCat
 
Yes it is. Radically so. It’s much more like a bat’s than a bird’s.
 

I was going to suggest it was her cutie mark, that somehow shows up when being electrocuted… seriously, people don’t become translucent when they get electrocuted, static or otherwise, it doesn’t make sense. But then that picture of ABs skeleton with an apple… I, uhh… did they change what her cutie mark would be? maybe?
 
I think the idea that it’s where non unicorn magic comes from, a ‘magic bone’ that others have suggested makes more sense, but that’s a terrible name we need a better one >.<

That’s cotton candy. She ate a lot before the show.

It’s her funny bone.

@Bigshot232  
The bone structure of her wing isn’t that different from a bird. Feathers are nestled in skin and muscle, not bone. Are the hairs of your scalp connected to your brain?

Cloud bone for pegasus, but can i direct your attentions to her wings. they are hole bones all the way to the tip of the feather, so…
 
https://derpiboo.ru/294907?scope=scpe3b3021f8ef028f2d97d19d2e36a43b624db5d060
 
Did apple bloom pull out a bone from scoots?

We saw an apple in Applebloom’s stomach when she got zapped. Maybe it is connected to their Cutie Marks.

That is not cloud that is her brain is there because she always goes with her gut.

@Ethaes  
Holy macro, ¿is that an apple on her tummy? ¡¿HOW DID I NOT SEE THAT SOONER?!

I bet it’s their Squeedlyspooch.

Is it too simple an explanation that she just downed a mouthful of cloud on her way through?

@Daneasaur  

That’s what I’m thinking. This has been my headcanon ever since the whole Tirek thing happened.

That’s actually really cool.

Her last lunch at Taco Bell?

I never noticed this!

Maybe all pegasi have a magic generating organ. Heck, maybe all ponies have one it just manifests differently depending on the type.

a cutie bone

Ikr? It’s like some random people thing you would see in something like psg

could be horse apples

That’s the weather bone, it helps controlling the weather…

A cloud. Applebloom has an apple.