My personal opinion of Kopa is: He was born after Kiara and Kion, maybe after Kiara decided to split the territory due to the two prides (maybe too many lions for the area?) OR his stories are just a confused history of the Pride Lands. Like, someone years after Simba, Kiara/Kion etc. is telling the story, and can't remember who the events actually happened to. OR he just doesn't exist at all, much like Ruffy, Fluffy and Scooter with Scamp in Lady and the Tramp.
I kinda like the idea of a split yet still jointed territory. Maybe the outlanders returned home after they let it heal and animals can live and graze peacefully there?
I personally like the idea that Kopa is Kiara’s son with Kovu. He still makes his appearance after Kiara joins the split prides but isn’t directly related to Simba.
Maybe that too.
