I don't have much of a theory for Mheetu, other than the fact that he was Nala's younger brother and that he didn't appear in the film. Ok, that's not a theory, it's a fact.
Perhaps, though, he was born after Kopa. Sarafina had Mheetu after Nala returned with Simba. He is uncle to Kopa but younger than him. I've drawn him quite muscular too.
Like this: [link]