Update: Well, color us surprised--nay (ha!), shocked. Turns out the horse's body was not a victim of photoshop after all.
American Apparel's creative director Iris Alonzo was kind enough to get back to us to explain that it's merely the angle and cropping of the image that makes it appear as if the horse's body was not there. And she attached the original, un-color corrected image as proof. Who would have thunk it?
The case of the missing horse's body has been closed. Thank you American Apparel for helping to solve the mystery.