As far as Disney goes, they first planned male cub to be in second movie. Then they added female, and most of story turned around female cub, so male was removed. Since he did end up with almost no role in story. Also as point of interest, cub at the end of TLK had few other names before Kopa.

Whatever else you heard, is fan fiction. Since no reasonable explanations were given by Disney, fans just made up stories. Because most of us were not really happy to hear that the cub at the end of TLK is Kiara... it just does not look like it. Even ceremony at the end of TLK and beginning of TLK 2 look different, which makes some of us believe that was not same ceremony and some unknown amount of time passed between two movies. Beside that some of us read books before TLK2, and had him for a cub at the end of TLK. So different reasons, but same result. A lot of fan fictions. Because even if the male cub at the end of TLK is not Kopa, than who is he?

Meanwhile others just take him as spin off character and do not place him in official TLK universe.

So to end it, it is up to you to make your own opinion about Kopa. There is most likely Disney will never give us any explanations.

Ok but I heard from a monitor that disny just did not have time to add him in but he may still e real!

Kopa never existed. The book series Kopa was featured
in is separatefrom the films, like an alternate universe if
you will. Kopa is not a 'real' lion king character. Fans may
theorize whatthey will though. Kopa is fun and cute to
make up storiesabout, but most of them are just fan-made.