We first met the great Kahless in "The Savage Curtain" during the third season of The Original Series, when he was played by Robert Herron. In that appearance, Herron lacked the ridged forehead worn by Klingons ever since Star Trek: The Motion Picture. We rationalize that the Kahless in "The Savage Curtain" was based on an image in Kirk's mind, and thus did not look like the real person. The Kahless in "Rightful Heir" in Star Trek: The Next Generation , played by Kevin Conway, wasn't the real thing, either, but at least it was a clone of the great man, so he presumably looked pretty much the same.