Kopa is a character from the world of The Lion King. A Lion cub, he was featured in the book series The Lion King: Six New Adventures as the son of Simba and Nala; as such, he is believed by several fans to be the lion cub at the end of the first movie; Kiara looked different than the cub featured there when presented in Simba's Pride. Kopa looks identical to the appearance of Simba when he was a cub, except Kopa has a light brown tuft of hair in the middle of his head. A favored story explaining Zira's exile and Kopa's disapperance is that Zira, overcome with grief, rage, and the need for revenege, killed Kopa. Simba, after hovering over his son's dead body, banished her along with the rest of Scar's followers.
Disney has confirmed that the cub seen at the end of the first Lion King film is not Kopa; as even though Kopa already existed in the books, the film makers of The Lion King said on the film's commentary track from 1995 Laserdisc and 2003 DVD releases, that the cub in the end does not have a real name or gender. They called it "Fluffy" and didn't give it a gender.
Many fans like to think that regardless of what the cub's creators have said, the seuquel's makers would've meant Kopa to be in the backstory of their film. But in fact, the sequel's makers made it impossible to be Kopa: Kovu's time of birth was before Simba returned to fight Scar, while Kovu's age in the cub scenes appears the same as Kopa's in the books. Kopa could not have been born back then because Simba and Nala hadn't even reunited yet. And there was too many months between Kovu's and Fluffy's births for them to look the same age. Also, if Kopa still somehow miraculously existed in the sequel film's story, he would've had to show in the film as according to the books' fact that he was the same age as Kovu, he would've been alive and with his family between Kiara's ceremony and the day she met Kovu. But even so, his existence would be contradicting because the books don't even mention Kopa to have a sister. All those facts make the timeline so that it's simply impossible for the cub at the end of the first film to be Kopa and for Kopa to exist at all in the film's story. Thus, if anyone wants to imagine the cub at the end of the first film to be Kopa despite of what its creators have said, they have to completely ignore the sequel film in order to make sense at all.
Kopa wasn't even created, written or developed by Disney at all, which is likely the reason why the cub in the end never was Kopa and why he wasn't taken into the movie universe even later. The first book in the series, A Tale of Two Brothers wherein Kopa first appears, is an original story created and written by the writer Alex Simmons, who isn't part of The Walt Disney Company. Alex Simmons' personal web site, provides that information.