In this article, I will try to provide you guys with a basic undestanding of the many cubs of Simba and Nala. Not all are official TLK characters, not all are unofficial. Please keep your various fan theories away from this article as this is not the place for them.

Fluffy
Fluffy is the genderless cub at the end of TLK, it's genderless because the film was predicted to be a box office flop so no sequel was ever planned. That didn't turn out the way Disney thought, huh? With the creation of TLKII: SP & TLK: SNA the cub could be either male or female depending on which cub you prefer as the de-facto child for Simba & Nala. If we're going by the canon timeline, Fluffy is almost certainly Kiara. If it's semi-canon timelines, Fluffy is Kopa.

Tanabi
Tanabi is probably the most well known of Simba's cubs, Tanabi is simply a creation of a fan who decided to write a very good fan-fiction after the end of TLK. Tanabi is simply fanon, not canon or even semi-canon. Tanabi first appeared in The Tales of Tanabi and later in, Chronicles of the Pridelands.

                                          
Kopa
Kopa, meaning heart in Swahili, is Simba's first cub with his consort, Nala. Kopa, whilst believed to be a Disney creation, was in fact created by an outside company which decided to cash in on the fame of The Lion King. Many theories exist on why Kopa was not seen in Simba's Pride and the most popular one is that Zira murdered him as revenge. The reality of it? Kopa was a fan-fiction character created by Alex Simmons who created original stories (aka fanfiction) for popular Disney works which were published by Grolier Enterprises. Disney did not acknowledge the stories, but simply allowed them to be published, after all, what could a small company do to Disney's reputation? As to why Kopa is not seen in the sequel to TLK, Disney did not see him as an official character. Kopa wasn't replaced by Kiara or killed by Zira. He just simply doesn't exist in TLK canon. He is nowt but a semi-canon character, an alternate universe character. As are, technically, any and all other characters mention in the Six New Adventures series.

Shani/Aisha
Shani or Aisha were working names for Kiara during production of Simba's Pride. She later became Kiara.

Chaka/Naba
Chaka or Naba was supposed to be Shani's brother and littermate during production of Simba's Pride as Simba was supposed to have two cubs. But Chaka was then axed and Shani changed to Kiara.

Kiara
In canonical events, Kiara is the true first cub of Simba & Nala. It is believed that she is the cub shown at the end of the original movie, and the sequel makes this official, all be it with a few design changes. Contrary to popular belief, she did not replace Kopa as Kopa was never recognised by Disney an official child.

Kion
Kion is the canonical second cub, and first son, of Simba & Nala. Another popular theory floating around in the fandom is that he, too, is Kopa's replacement. As stated previously, Kopa is not recognized as an official cub of Simba & Nala so he cannot be Kopa's replacement.