Nala's father is a character who is mentioned in The Lion Guard.
He is the father of Nala and thus Kiara and Kion's maternal grandfather.
Contrary to belief, the directors of The Lion King did contemplate who Nala's father might've been. Albeit jokingly said, he was at least thought about as a character, but had no place in the movie. In the end, the two directors just hoped no one would notice his absence, effectively scrapping the character.
Some years after this incident, he mated with a lioness named Sarafina and had a daughter with her named Nala. At some point before or shortly after Nala's birth, her father either died or left Pride Rock, thus leaving Sarafina as Nala's sole parent.
Apparently, he bore a strong resemblance to his grandson Kion in his youth, as he was mistaken for Kion in the paining.
Minkoff (right) and Allers (left) caricatured as lions in The Art of The Lion King.
Painting of Nala's father with the previous Lion Guard.
It is unknown whether he would be Mheetu's father as well, though he probably wouldn't as Mheetu was deleted by then thus couldn't be fathered.
In The Lion King 1½ trivia game, he is mentioned as an option to one of the questions.
In 2004, when asked about who is Nala's father, according to Brian Tiemann, Minkoff, and Allers "began laughing, ribbing each other, making little sidelong jibes". Eventually, Minkoff pointed towards Allers, jokily mentioning he was the father. They concluded their answer with Minkoff muttering into his sleeve, that the general assumption was that Nala's father was "either Scar or Mufasa".
According to Tom Bancroft, one of the lead animators for The Lion King, the issues of Nala's father was brought up very late, and eventually ignored as they figured no one would notice him not being there.