On the actual topic, because I do find it interesting (not the is kopa official thing, but who is the cub). You are right Hasira, that it is just Simba and Nala's cub. However because of the creation of Simba's Pride, it opens up a whole new realm of possibilities. Because the cub has a different design, you can then claim that it's not the same cub, because they could have gone back to see how they drew it to copy it, but they didn't. They made her unique, and perhaps a little older looking.

So therefore you do have the option of saying that it's possible that they had a cub before Kiara and that cub could possibly be Kopa, it could be cub Xyz for all I care, but I'm just saying that the differences allow for options in terms of the fandom and how you want to look at it