So many of these start out as such simple drawings...
I admit, I love fair Rarity. Pound for pound, I find her the most entertaining pony (although one shouldn't measure a lady by her pounds.) It's no real secret that my favorite of the characters is Twilight, but I can't deny that Rarity steals most any scene that she deigns to enter... doubly so if she sings. (I am among the many commercial artists won over to the series by her excellent ballad to the headaches of pleasing clients in "Art of the Dress".)
I'm consistently shocked that she meets with more than her share of disapproval from certain fans, as a result. How diminished the show would be without any one of the Mane Six, no doubt... but how truly drab things would be without the pony every pony should know.
Edit: After some complaints that she looked too old, I gave her the ol' Hollywood cheesecloth treatment and did a softer blend on the brushmarks making up her face.