Drawn on August 18th, 2013.
Artist's DeviantArt account deactivated. Original source link not yet recovered at the time of upload.
Name of the changeling is Colonel Gel Nexus (owned by EnigmaMystere), and for the green unicorn, Rhino Nexus (owned by Rhino). The white unicorn mare is Gel's default disguise, named Angelic Grace.
This is a recovered high-resoltion file. A low-res duplicate was uploaded before as >>2680787:https://derpibooru.org/images/2680787.
Read the fanfic by Rhino and EnigmaMystere: An Alternate Path