I always thought it was a brilliant move on the part of Mattel to have Kobra Khan join the Snake Men when those particular characters were introduced to the Masters of the Universe toyline. It gave the character a greater importance, especially when Skeletor had instructed him to be a spy. Even though this sub-plot was used in numerous canons, it was never fully utilized to its full potential. In this panel from the minicomic "King of the Snake Men" by Bruce Timm we see the moment Skeletor orders Kobra Khan to join the Snake Men.
(click on the image to see it at full-size)