The chick at the end of the film is Fluffy, and Kopa was never an official character in movies, books maybe, but never in the movies.
Kopa's theory is that the universe had no Kiara him, only he had, and I hear Disney did not even know when the books began to make The Lion King: Simba's Pride.
So we Kopa never actually existed films.

Thanks for answering. The theories make sense to me so thanks for your help. Using them I have come up with my own explanation so again I thank you all.

PART 2 OF MY MESSAGE:
A bit easier placement of whole thing is if you put Kopa behind TLK 2, since you have more room for different explanations, why Kiara and Kovu are not there. From one that they simply started new pride somewhere else after Kopa was born. Could be that in laws of Pridelands, first entitled to be king is direct male descendant. Which would put Kiara behind newborn Kopa in line to inherit kingdom. Which could trigger an reaction of them follow idea form second movie and form new pride somewhere else. What this theory removes is reason why Zira and rest were exiled to outland, just because they supported Scar sounds to me bit too weak reason for not giving them another chance instead of exile.

Some theories just have holes, and there is really hard to fill them up. But as long as you take them as fan made, I guess if you try hard enough you can fill all holes somehow. Also I made these out quickly, so dunno how many holes they have.... but there is no real, final or official explanation for it. According to official movie universe there is no Kopa. But I still take him as part of TLK universe, one of those Disney never bothered to connect to rest of universe. So when and where to place Kopa, you have totally free hands and each theory has its pluses and minuses.

This really depends on who you listen to. I mean yeah, cub at the end of TLK is Kopa in book universe and at same time it is Kiara in second movie.

Now who was first is real question. As you pointed out, Kovu can't be much older than Kiara. Yet he was hand chosen by Scar, who he never really met, which means it must have happened after birth and before Simba came back. Which gives very little room for Kopa to happen between that. Now one could theorize that Scar chosen Kovu before birth, which would give it more time and if Nala got pregnant in that jungle with Kopa, than that would give it some time, when Kopa could be born before Kovu, and got killed by Zira soon after birth. Which than resulted as exile of pregnant Zira, while Nala got pregnant for second time. That could make small enough difference in Kovus and Kiaras age to not be too noticeable.

Problem of course is Vitani, since she is similar age as Kovu, which in my opinion makes them twins. So how could Zira decide which one did Scar choose? I guess she took him literarly when he said king and so she knew he meant Kovu and not Vitani.

Kopa is ment to be the cub seen at the end of TLK 1, the time difference between TLK 1 and 2 must be at least 6 months as Kopa was born first and never interacted with Kiara. Nuka had already been born in Scar's reign and If Kiara was born 6 months after Scar's death if we are asuming that Kiara and Kovu are the same age than Kovu could of never been born before Scar's death, my theory is that Scar thought Nuka was to weak to be the heir so when Scar was being eaten alive or whatever asuming Zira was there; Scar could of made a promise to her that the next male cub she consievs would be the chosen one , it kinda would make sense because If Kovu was already born the age difference between Kiara and Kovu would be a lot. Zira was probably exaggerating when she said hand chosen as it even says in the movie that Scar isn't even Kovu's real father.

Sorry if I'm confusing u, I'm confusing myself XD

So Kopa was born when Simba and Nala had become king and queen, Simba and Nala might of waited untill Kopa was an older cub and then got pregnant with Kiara that would meen Kovu and Kiara's age difference would be buy a couple of months.
There is this theory that Kopa was originally betrothed to Vitani that would make Kopa around the same age as Vitani and I personally think Kovu is younger than her.

Though Kopa only grew up to be about the same age as Simba was when he was a cub as Zira wanted Revenge on Scar's death so she took Kopa's life. Simba was furious and exiled Zira and the outlanders to the Outlands.
I'm guessing after Kopa's death Nala must of been pregnant for about 7 months based on the Vitani/Kopa theory witch would mean that all cubs would be roughly months apart from each other..

I'm rubbish at explaining things, but good question