Nice work!
Had some fun experimenting on your twilight.. (lol that sounds kinda sinister)
First thing I changed was the eyes, they should be more slanted, in opposite directions, to follow the head. \ /
Also, nostrils (or what it's called on a horse) same shape there \ /
Is the head supposed to be looking upwards? If so the mouth and nose should probably be moved further up.
Changed the colors a bit to be less saturated and more pastel-y.
And I played around a bit with some AA and shading.
I also removed the noise in the hair, personally I prefer it clean, especially if it's ponies.
I didn't dare to mess around with the hair too much though, it seemed rather intricate and complex.
edit:
I tried matching the color from the show more accurately.
Seems like the difference was way more radical than I had thought.