Kopa is Simba and Nala's son from a book called Six New Adventures
People are debating whether or not he's the cub at the end of TLK, and if so, why he's not in SP and what happened to him etc. One popular theory is that Zira killed him and was banished from the Pridelands along with the rest of her pride not only because she was still loyal to Scar, but for the murder as well.