Mufasa: "Simba... Let me tell you something that my father told me... Look at the stars. The great kings of the past look down on us from those stars." Simba: "Really?" Mufasa: "Yes... So whenever you feel alone, just remember that those kings will always be there to guide you ... And so will I."
—Mufasa and Simba
The Great Kings of the Past, according to Mufasa, are deceased rulers of the Pride Lands, who look down on the living beings from the stars. While it is commonly believed that the lions in The Lion King believe the stars themselves are the Great Kings of the Past, it was never actually confirmed in the film.
"You mean a bunch of royal dead guys are watching us? Pbbb."
—Timon mocks the Great Kings of the Past
After Mufasa briefly plays with Simba after scolding him for his disobedience, Simba cheerfully tells Mufasa that they will always be together. However, Mufasa tells Simba about the Great Kings of the Past, who are former monarchs of the Pridelands who look down on the living. Mufasa later tells his son that when he feels alone, the Great Kings will always be there to guide him, and so he, one day.
The Great Kings of the Past are mentioned in the film again by Simba when he, Timon, and Pumbaa offer up their theories of stars. Timon mocks what Mufasa told Simba, and indirectly calls Mufasa "a mook".
Kovu: "Ah, I've never done this before." Kiara: "Really? My father and I used to do this all the time. He says all the great Kings of the Past are up there." Kovu: "You think Scar is up there?"
—Kiara and Kovu stargaze
After the rhino chase, Kiara and Kovu stargaze later that night. Kiara sees a constellation of a baby rabbit while Kovu sees a constellation of two lions fighting each other. Kovu enjoys this pastime, and states he's never done this before. Kiara states the great Kings of the Past are up there, indicating Simba told Kiara of what Mufasa told Simba when he was young. Kovu responds by wondering if Scar is one of the Great Kings of the Past.
In the stage musical productions of the original film, Mufasa sings "They Live in You" when Simba asks whether he and his father will always be together, causing Mufasa to look up to the night sky. He tells Simba about the Great Kings of the Past, and how they guide and watch over everything. Again, the Great Kings of the Past are mentioned when Simba, Timon, and Pumbaa look up at the stars, wondering what they are. Simba recalls Mufasa's words regarding the great kings of the past, but Timon and Pumbaa dismiss it and laugh at the notion.
Mufasa was seen in the clouds, coming to Simba in his hour of greastest need in the form of a ghost. He reappears in The Sequel for his Granddaughter's birth and suggests to Rafiki pairing up the Outlander prince and Pridelander Princess.
Mohatu was revealed to Simba through Mufasa, never appearing as a ghost but a bright star.