Like I said in my comment, I really like the idea in this. It's Rainbow Dash, soaring amidst the clouds with a strangely long tail... that, combined with the smaller cloud, forms her cutie mark. Very stylish and clever!
The technical side isn't anything special: it looks good, but not awesome. The clouds are nice and reminiscent of the show's style, but RD herself isn't drawn in an especially interesting manner. Not bad, but I'd like to see you develop an interesting, original style. Then again, maybe that's just me; I prefer more natural, "hand-drawn" looking styles.
The colours are quite balanced and the composition is good, drawing the viewer's attention to the "hidden" cutie mark. The larger cloud works nicely as a background for the pegasus. Overall, I'd say this is a very successful image in what you were trying to achieve.