Genghis Khan was a historical figure in ancient Earth history.
In 2266, Korby believed that by transferring consciousness into an android body, a Human would have practical immortality. James T. Kirk responded that this was programming. He continued further by saying that others had made the same promises, using different words, and that these others were Genghis Khan, Julius Caesar, Adolf Hitler, Ferris, and Maltuvis. (TOS: "What Are Little Girls Made Of?")
This character was only mentioned in dialogue.
The Excalbian appearance of Genghis Khan was played by actor Nathan Jung.
His title meant "Greatest of All Rulers." Genghis Khan's actual birth and death years, circa 1162 and 1227, were noted in the Star Trek Encyclopedia (4th ed., vol. 1, p. 302). He conquered much of Earth's Asian continent.
The initial appearance of the Klingons was partly inspired by actor John Colicos' suggestion that the make-up artist make him look like "a futuristic Genghis Khan" for his appearance in "Errand of Mercy". (Star Trek 30 Years)