@CogWeaver  
aka like a drill bit but with a lot more tapering.

It looks like she’s got ravon wings

@lunafan22  
It’s not, it’s supposed to look like it’s spiraling upward. When she opens her eyes in the gif, you can see the lighter parts curving upward in the same direction around the horn

@lunafan22  
Going by the comics, the changelings were creatures born of dark magic. {Along with several other things.} So it could be that dark magic on regular beings gives you a sort of changeling-esque form or part.

but why she has chrysalis horn?

I want to see the part where Rainbow pile drived the plant into the ground like a badass.

I do like the reoccurring visual they use in Equestria Girls where when a person is overtaken with uncontrollable / dark magic and before their new evil personality takes over, they make it a point to show the character’s tears evaporate into mist.
 
For Sci-Twi they don’t show that here, but in an earlier part of her transformation they do.
 
It’s a second or so after she pitifully begs for someone to help her.

…
 
 
The Deja vú is strong in this one

@ThisGuyHasNoName  
Forceful demonic transformations tend to be.
 
Just ask Sunset!

I’m not sure, but it looked kinda painful at the start….