Surprisingly few have heard about Fluffy and many of those who have, are confused with all the books and the sequel and stuff. Here's the story digging into facts of the timeline and looks, as well as the official words by the film's makers. I originally created this article to The Lion King Wiki as SuperBabySimba, but as Wikis can be edited by anyone who is confused and prefers daydreams over facts, I chose to post this original version here to help those who are interested. :)

FLUFFY'S STORY

"Let's just end on the next generation. Here's Fluffy, Simba and Nala's new little cub. Let's just end on Fluffy."
—The Lion King film makers, on the 1995 commentary track of the film, refering to the cub at the end.

Fluffy is what the cub at the end of The Lion King film is officially called.

Appearance

Fluffy looks a lot like its father Simba did as a baby. It has very similar facial features, golden brown fur, paler toes, and big, black pupils without irises. But couple of noticable differences can be found: it has black ear rings whereas Simba didn't and it doesn't have fruit juice 'paint' on its forehead unlike Simba did.

Information

After The Lion King

After the release of the film in June 1994, the cub became very popular object of speculation as for who it is. The film's content itself doesn't say anything about the cub apart from its outside appearence. No name or gender is given. Because of the look-alike appearence to Simba and how no female had been seen with a golden brown fur, many believed it to be a boy and many still refuse to believe it's not.

The ear rings make some absolutely certain that it's a boy, even though Simba as a baby didn't have them and Simba's mother Sarabi does have them, which supports the idea that Fluffy is not a boy. And  the fruit juice paint was an essential part of Simba's blessing ceremony in the beginning, the lack of it at the point where the only known baby boy had it, implies that it might be a girl - especially when combined to the fact about the ear ring matter.

Thus, the original design is genderless by looking like a boy but having couple of very crucial design differences. Some outsider writers created childrens' books wherein it was visioned as a son. In 1994, few months after the film, was published book series The Lion King: Six New Adventures, which became the most popular but remained quite unknown as it was released only in USA. In it, Simba and Nala have a son named Kopa. Those books have never been tied into the film by its makers and thus are not officially semi-canon but just official fanfiction.

As in 1995 the film's creators released a commentary track for the Laserdisc release of the film, wherein they called the cub "Fluffy" and didn't refer to it either a 'he' or 'she' - even though Kopa was in Disney's copyrights and there would've been no harm done by turning the cub into him or confirming it a gender. By Fluffy, they made clear they didn't want the book universe connect, that Kopa is not the cub in the film itself and that in the official story the cub would remain without a gender or real name.

Soon enough Disney's film makers started to create an official sequel film, but the crew was all different from those who created the canon film. A rumour has it that the sequel's creators were planning to write two cubs, a boy named Chaka and a girl who had several name options, but cut the boy out in early production and developed the story only with the girl. Any official source for them ever considering a boy cub has not been provided, that's why it's only a rumour. In 1998, The Lion King 2: Simba's Pride film was released and Fluffy had finally gotten an official movie universe gender and name: a girl Kiara.
 

After The Lion King II: Simba's Pride

They used Fluffy for baby Kiara in the film's official trailer which ought to make very clear Fluffy and Kiara are supposed to be the same cub in essence. But even so, as Kiara's design and the ceremony are obviously different from the first film's version in the actual film, it has caused confusion among fans - especially those who know about Kopa in the books. There are a few fan theories to connect Kopa into the story, of which the most popular is that Zira killed Kopa before the film's beginning despite of the fact that a loss of an earlier child doesn't make sense to about a twenty facts in the film's story.

In the film's official trailer they used Fluffy for Kiara's presentation, instead of their redesigned version. My guess is that they needed to connect Fluffy into an official SP material but in the actual film they wanted to redesign it, so the solution was to make the point in the trailer. And yes, even if the trailer was made by the marketting people, The Walt Disney Company does see this trailer as official as it has the word "Disney's" on it and it was released on an official Disney VHS. Also, Kovu's animators has confirmed in a magazine interview, that Fluffy is Kiara.

And those who correctly do their math on the character timelines the two films combined provide, find out that it doesn't even leave room for any older siblings to Kiara, the very least for Kopa: Kopa from the books and Zira's son Kovu from the sequel film look the same age (and thus would become the same age if stuffed into the same story) yet Kovu was born before Simba even met Nala, and Fluffy could not have been born before in the least 3 months later. This could have Kovu and Fluffy look the same age. Which Kovu and Kiara do. If there was another cub before Kiara, carried while Kovu growing up and then growing up alongside Kovu, then Kiara carried and growing up for months...Kovu would look much older than Kiara. There is no room for anyone between Kovu and Kiara's births. Especially not someone as old as Kopa.

Why they remade the cub's ceremony scene is unknown. Why they redesigned the cub is unknown but likely they wanted it to look more obviously female, or just less like Simba as in more personal and individual appearence instead of a clone of her father. Whatever the motive, the act is retroactive continuity that is very common in works of fictional series.

One essential thing remains common between Fluffy and Kiara and the ceremony scenes: neither have the fruit juice paint on them when Rafiki holds them up for the kingdom to see. Kiara gets the paint after Rafiki takes her down. Thus, the continuity doesn't completely lack and Kiara and Fluffy blend into one on a practical level in the films too.

Simba's Pride's makers have officially confirmed that in their story Fluffy is Kiara. In this official interview they say "It starts off where the first film left us with a twist - you think it's a king but it's a queen!" And remark also that he says that we THINK it's a boy, not that it actually was in the film itself. But the point is, one of the animators have confirmed that in their story Kiara IS the cub at the end of the first film.

Fluffy is Kiara in the entity of the movie universe, which forms the official story of The Lion King, while the first film's commentary track proves Fluffy never was even supposed to be officially anyone until Kiara.

Fluffy = Simba and Nala's first child because its creators said so.

Fluffy = genderless Fluffy in its own film's story alone. Thus turns into any fan's original character.

Fluffy = a boy in the book universe alone. As the book universe stories contradict each others, Kopa isn't even the only version of Fluffy in the book universe.

Fluffy = girl Kiara in the entity of the official story that is the one formed by the two films.