Outsiders (also known as Outlanders) are an offshoot of Pridelanders. They serve as the quaternary antagonists turned supporting characters of the film.
The Outsiders are a pride of lions who remain loyal to Scar after his death. Because of their ties to the former tyrant, Scar's nephew Simba banishes them to the Outlands, where they remain until the pride is disbanded, and its members, except their leader, Zira become Pridelanders again.
When Kiara, the young daughter of Simba and Nala, expresses interest in romping the Pride Lands, Simba warns her to stay away from the Outlands, where the Outsider pride dwells. Overhearing the conversation, Zazu flutters up to the king and warns Kiara that nothing inhibits the Outlands but a "bunch of backstabbing, murderous Outsiders." Simba then warns his daughter to never turn her back on them.
The first Outsider seen in the film is a young cub named Kovu. Upon first meeting him, Kiara takes her father's advice, never turning her back on the stranger. Intrigued by her behavior, Kovu questions her and then mocks her for always following her father's rules. He then boasts that an Outsider is self-reliant and can take care of himself.
Not long after their encounter, Zira shows up to defend her son, and Simba jumps up in his daughter's defense. The two have a stand-off conversation, during which Zira laments the poor conditions of the Outlands, where her pride is deprived of food and water. Unsympathetic, Simba reminds her that an Outsider is to be killed if he strays into the Pride Lands. Testing Simba, Zira pushes her son forward, but Simba takes pity on the cub, kicking Zira and Kovu out of the Pride Lands.
Once home again, Zira yells at Kovu for associating with a Pridelander, reminding him that Simba has banished their pride from the Pride Lands for staying loyal to Scar. She then gets a marvelous idea to make an older Kovu woo Kiara in order to gain Simba's trust and then kill him. She then sings "My Lullaby", during which her vengeful pride echoes her murderous intentions.
Years later, the plan goes underway, and Nuka and Vitani set fire to the Pride Lands, making way for Kovu to save Kiara and wedge his way into Simba's pride. While the Outsiders wait excitedly for Simba's downfall, Kovu begins to fall in love with Kiara and refuses to kill the king. Sensing Kovu's betrayal, Zira and her pride ambush Simba, leaving him severely wounded. Though Simba escapes, Nuka dies during the attack, leaving Zira in a fury.
The vengeful lioness leads her pride into battle against the Pridelanders, and the Outsiders exert well-honed battle strategies and skills. Despite the ferocious tussle, the fight is soon stopped by Kovu and Kiara, who convince Simba to treat the Outsiders as one with the Pridelanders. First Vitani, then the other lionesses see Kovu and Kiara's point and Zira's true nature, betraying her and coming to Simba's side in favor of peace and a better life.
After Zira's death, Simba welcomes the Outsiders back into his pride. Once back at Pride Rock, the Outsiders turned Pridelanders witness the union of Kovu and Kiara. They then bow their heads as first Simba and Nala, then Kovu and Kiara stride past and roar out over the kingdom.
The Outsiders appear in the episode "Lions of the Outlands". They originally lived in the same part of the Outlands as Jasiri and her clan and refused to share a waterhole with them, until Kion used the Roar of the Elders to blast them to a different part.
The Outsiders' alternative name Outlanders is the name given to the other residents of the Outlands such as hyenas, jackals and vultures in the 2015 TV film The Lion Guard: Return of the Roar and its 2016 follow-up series The Lion Guard (both of which take place in the middle of Simba's Pride).
The Lion Guard reveals that the Outsiders, like the Pridelanders, are enemies with hyenas, regardless of their unawareness that Scar was killed by hyenas. It also squares with the idea of Zira originally being accompanied by the hyenas, but instead outsider lions in The Lion King II: Simba's Pride.
Outlanders fighting Pridelanders
The Outlanders and the Pridelanders come together as one
The Outsider lionesses
Outsiders turns good because they are convinced by Kovu and Kiara and discover Zira's true nature
Two Outlander lionesses (right) and a Pridelander lioness (left)
Outsiders are frightened that Timon uses Pumbaa's tail to shoot gas