Twilight set her blanket ablaze with her Rapidash powers! Oh crap, quick, get the fire extinguisher!

reminds me of Prequel

now think she in that form casting dark magic…..that cant end good at all

Ha ha, that’d be an inconvenient power to have.

Like most young fillies. Twilight used to wet the bed, now she has an incinerating problem.

@BP  
Headcanon accepted

That is my personal theory, at least.

Fan Theory: The Fire Mane is a result of her excess magical energy growing out of control when her emotions are both unstable and angry. It probably happened off screen again when she blew her tree house 40 feet into the air.
 
Since the source of the elements, i.e. Rainbow Dash, infused Twilight with magical power, she had absolutely no control over it because, since will=level of magic, Twilight’s will is miniscule (but growing) compared to Rainbow Dash’s, which was enormous since she was young. Since unicorn magic only happens when you make it happen, it means Twilight’s energy is not all her own. This is what happens when you put a power source within something that can’t contain it.
 
As Twilight trained under Celestia, this became less and less of a constant problem. The plus side is she has a nearly infinite source of magic, enough so that she can teleport nearly a dozen times by being lazy. The minus side is that it is slowly degrading her sanity and her ability to control her anger.

Let it go, Twilight.

Now that’s what I call a fire crotch.