I recently finished one of the most downright enjoyable commissions I've ever had the pleasure to be assigned. Some time ago, Lucia Cochrane-Davis asked me to help her visualize her birdlike fantasy race, the Icariids. She gave me a wonderfully detailed description, and the creature design eventually wound up being a full-on species sheet. Here is a montage of the many sketches I drew during the process:
Montage of Sketches for the Icariid Species Sheet by ~babbletrish on deviantART
Looking back on these, it's mostly the facial design that got altered the most. Aside from that, I liked the woman in the lower-right ever since I first drew her and she's in the final version as well almost exactly as she appears here; I just needed to straighten out her tail feathers.
As a bonus, here's the mockup of the final species sheet, complete with all the spelling errors and formatting snafus: