@Liggliluff
 
Yes, it does change pretty constantly. I found myself redrawing that thing several times trying to reconcile where it’s supposed to fall on the hips, under the tail, and over the crotch.
 
In the other image, I realized that if you just aim for a ‘level’ waist, the dip happens all on its own due to the natural roundness of the region. An optical illusion, if you will. The key was to stop tailoring the undies and just let her fill out a pair of bargain-brand underpants. At least, that’s what makes sense in my own head ^^
 
Some other things worth noting about Apple Bloom and her undergarments:
 

@Trefoiler  
The panties in the animation changes a lot in each frame. I think it looks to dip down more as she’s leaning forward (as how scrunched up the panties is from the side), but the dip reduces as she stands up.
 
I have estimated the dip to the crotch is 20% (80% the height) compared to the sides to the crotch. Your drawing looks more like 50%. Then there’s this frame >>862521 where it looks to be a 30% dip (70% of the height)
 
—
 
I’ll assume the bandeau comes with fake boobs.

@Liggliluff  
Actually, they’re identical!
 
And as for the bra… well, the source material is kind of funky on that point. She’s flat until she puts on her costume, magically filling it out somehow.
 
>>862245p
 
It’s probably not unthinkable that it’s a wonder-tube-top, but I eventually decided that my Apple Bloom has been a magical girl for a few years now and doesn’t need the padding anymore. ^^
 
Besides, a hero needs an undergarment that tucks easily into her costume. Ironically enough, lingerie would be the most… practical choice?
 
 
_{Did I just describe lingerie as practical?}

I think the panties goes down a bit too far on the front. She also doesn’t seem to have any boobs in the original, so I would suggest a cupless bra. A training bra.
 
But I do really appreciate seeing underwear variants among clothed and nude version.