He's not in TLK:SNA, the others are incorrect. He is actually a character designed by the film makers, but deleted in the early stages due to time restraints. He was intended to be Nala's little brother, but in "The art of the lion king" he is mentioned as Simba's little brother.
Originally, the plan was that Simba would be saving Mheetu from the gorge, and thus being almost killed in the wildebeest stampeded. Mheetu is pronounced as "Me too" and this was his only line in the film. When Nala and Simba scurried off to play, he says this as he wants to join them.
Mheetu was designed to give Nala someone to protect during Scar's reign as king, but of course, when it was decided that their would be no idea of Nala's cub/teenage years during Scar's reign, it was pointless.