This theory is very popular and for a good reason, but this theory states that Zira and her pride were originally rouges who entered the Pride Lands after they lost their king. They found the Pride Lands, where Scar took pity on them and allowed them in, but he didn't know she was expecting. Vitani and Kovu were born, and Scar assumed they were his cubs, so he chose Kovu to become the next king. That, or he knew they weren't his, but he couldn't kill them in fear of angering Zira, and they were technically born in HIS pride, making them HIS pride members. That was enough for Scar to eventually pick Kovu as his heir.
I think this theory makes total sense. In the film, we see no lionesses who resemble the Outsiders. They're all clean, well fed, groomed, meanwhile the Outlander lions are scrawny, weak, greyish, and they look terrible. I watched the films. Zira's pride has at least 16 members, but Mufasa's pride has at least 15-20 lionesses. The numbers would not add up if they suddenly went from Mufasa's side to Scar's side.
In addition, Simba would've known about these lionesses before he was banished, which would've made it harder to exile those lionesses, especially if he found out one of the lionesses he knew had cubs. Him being in exile while these lions were inducted would make Simba less hesitant to banish them, since they weren't originally part of his father's pride, and it would explain Nala's attitude in the second film, why she isn't that defensive of Kiara when the Outsiders send Kovu in to seduce her - She grew up with them when Scar allowed them in, so she finds it harder to be more hateful towards them.
As mentally unstable as she is, if she lived during Mufasa's reign, she would've been comfortable with that - Rather than Scar's reign, but if Scar's reign was a first-hand look at the Pride Lands, it would've made her more comfortable.
I mentioned how awful Zira's name was and how terrible her parents must've been. In no way would a Pridelander name his or her daughter "Zira" so that seals the deal for my theory.
In the final battle, we see 7 lionesses compared to the 15 + lionesses during Mufasa's reign. Maybe the dropping numbers of lionesses forced Scar to take a different approach and Zira entering the pride was a lucky option.
If the Outsiders weren't nomads, the only logical explanation for their appearance is that they would be Scar's cubs, since he would need to breed more lionesses to keep the population up, and Zira later "Adopted" them when Scar died, which makes sense, but if Scar chose a non-royal cub as his successor, why go through the trouble of having children from the get-go, and why not have children with someone who was clearly that devoted to him, if he already had cubs with half the lionesses in his pride, who most likely hated his guts?
Side note, but the theory that Zira is Scar's mate wouldn't make sense: In the films, we see that the Queen (whether by marriage or of blood) is the leader of the hunting party. If Zira was Scar's mate, wouldn't she be in charge of the hunting party and not Sarabi?
Also, Scar wouldn't let a random rouge seduce his lionesses, not just because of lion nature, but because if a lion outside his pride got on the good side of one of HIS [Scar's] lionesses, then that would allow said lioness to become corrupted by that rival male, and use their children to upstart the current king, essentially draining him of his pride. Scar wouldn't see rouge lionesses as a threat, so he'd be more accepting of them, and any cubs that came along on the way.
As for the final battle, we don't see Zira's pride anywhere. 5 years ago, I read a theory that very well ties the 2 films together -
The theory states that on the day before Simba's return, Kovu was born, and Scar had chosen him to become his successor. Zira's pride escorted Zira far from Pride Rock where she could recover from the bodily stress and where Kovu would be safe and healthy. The next evening, Scar is angered at Sarabi's hunting party for not finding enough food for his newborn heir.
When Zira recovers from the birth, she finds out that Scar was killed and Simba is the new king. Simba finds out about Zira and her pride from the lionesses, and he doesn't hesitate to banish them for their loyalty to Scar, not knowing or caring for the matter that Zira had cubs.
Mufasa wanting to unite the two prides makes so much sense - His brother allowed them into the pride, but they were stripped of that position and long to re-gain that position they were given to. The story would be better that way rather than them betraying Mufasa's loyalty to Scar, and then suddenly being angered at Simba for banning them.
TLDR: The Outsiders were allowed into the Pride Lands by Scar, banished by Simba, then allowed back into the Pride Lands under Kiara and Kovu.
