The Lion King (title)
The Lion King is the title given to the male ruler of Pride Rock and to his lion pride, meaning he is the alpha male. The most recent Lion King is Simba, but once he dies, the title would belong to Kiara, making her the Lion "Queen." The creators of The Lion King film have not revealed their official vision on how many and who were the great kings of the past about whom Mufasa told to Simba.
The semi-official book and comic universes have their collected lot of kings, but on the commentary track of the film, the film makers have made clear that those kings never existed in the movie universe. That, and the fact that the books and comics were not created by Disney at all but are copyrighted to Disney, makes them semi-official and nothing more or less. The term "Lion King" is used in the book universe, in The Lion King: Six New Adventures when Kopa describes his dad as "The Lion King."
List of Known Lion Kings In The Official Movie Universe
Mufasa
The son of an unnamed and unknown lion. Mufasa ruled the land peacefully. He tried to spend time with his and Sarabi's child, Simba, but sometimes being king and a father clashed. A hornbill bird Zazu was his assistant. Mufasa was a wise and very powerful king, until one day his own brother, Scar, killed him and took over the Pridelands. Even so, the spirit of Mufasa appeared in the clouds to talk to Simba, first to remind him who he was, and then to congratulate him on uniting the two prides. Mufasa told the child Simba that the great kings of the past look down on us from the stars but it is not known how many of them were there or who they were.
Scar
Unlike the other kings before him, Scar took control of the Pridelands by force, killing his brother Mufasa and driving out his heir, Simba. Once he was the Lion King, Scar allowed the hyenas, who had aided in his scheme, to overrun Pride Rock; the result was a huge lack of food from too many hyenas greedily eating food and the lions barely scraping together what they could. Famine fell over the Serengeti until Simba returned to defeat his uncle Scar and take back his place as king.
Simba
After he defeated Scar, Simba took back his rightful place as Mufasa's heir and the Lion King. In a few months, the grass had grown back and the famine ended. Simba is a peaceful ruler, though when he had his daughter with Nala, his mate, he became a very overprotective father. During his time as king, with a little help from his daughter Kiara and her lover Kovu, Simba successfully reunited his Pridelands pride with the Outlands pride of lions who he had exiled earlier. With this reunion, greatly increasing his pride's strength.
Kovu
Kovu was not born a king, but fell in love with Simba's daughter, the princess of the Pridelands. It takes a while for Simba to accept him, but after he proves his loyalty, Simba accepts him into the pride. Kovu's succesion is assumed.
List of Known Lion Kings In The Semi-Official Book/Comic Universe
The book universe contradicts even itself, so this blood line isn't exactly valid.
Unnamed King
He is the first officially known alpha lion to dominate Pride Rock. All that is known about him is that he fathered three sons, making him Mufasa's great-great-great grandfather and Simba's great-great-great-great grandfather. In the Swedish comic "A False Ancestor," the phony "King" Joe pretended to be him to trick cub Simba.
One of the three sons
Just like when Scar isn't chosen to be King because he was scrawny and younger, two of the three brothers (the sons of the unnamed king above) couldn't have been chosen to become the Lion King. However, one must have been, so one of them was the dominant male; though in a lion pride, there can sometimes be two or three males, so the brothers may have stayed together.
Their son or daughter
Since one of the trio was Mohatu's grandfather, the Lion King out of of the three brothers must have had a son or daughter that would be the royal father or mother of Mohatu. Nothing else is known, not even their gender.
Mohatu
Little is known concerning Mohatu except what Mufasa tells Simba in the book "The Brightest Star". According to that, he settled disputes among the animals of his kingdom when a great drought arose and everyone was arguing to get a drop of water, and even some of his subjects' prejudices against other animals. Mohatu discovered the Great River, which appears to extend from the lake in the oasis Timon and the meerkats make their home around. He ruled for a long time, and died very old one winter’s night; his star is the biggest and brightest in the sky since he was such a great and noble ruler.
Ahadi
This king is Simba's grandfather, Uru's mate, and the father of Mufasa and Scar. It is thought that Ahadi was a bit proud, but perhaps the worst thing he did was neglect Taka(Scar) over the next Lion King, Mufasa. On the whole, he seemed to be a good king; for example, he rescued Rafiki from hyenas who were hunting for fun, a crime, introduced that monkey to be the royal adviser and shaman, and set up Zuzu's family to serve and report to him.
Mufasa
The son of Ahadi, Mufasa ruled the land peacefully. He tried to spend time with his and Sarabi's child, Simba, but sometimes being king and a father clashed. Zuzu's son Zazu was his assistant. Mufasa was a wise and very powerful king, until one day his own brother, Scar, killed him and took over the Pridelands. Even so, the spirit of Mufasa appeared in the clouds to talk to Simba, to remind him who he was.
Scar
Unlike the other kings before him, Scar (or Taka as he was previously known) took control of the Pridelands by force, killing his brother Mufasa and driving out his heir, Simba. Once he was the Lion King, Scar allowed the hyenas, who had aided in his scheme, to overrun Pride Rock; the result was a huge lack of food from too many hyenas greedily eating food and the lions barely scraping together what they could. Famine fell over the Serengeti until Simba returned to defeat his uncle Scar and take back his place as king.
Simba
After he defeated Scar, Simba took back his rightful place as Mufasa's heir and the Lion King. In a few months, the grass had grown back and the famine ended. Simba is a peaceful ruler who trustes the help of his friends and treis to do the best by his only child, son Kopa.