Mufasa: The Lion King gave fans an interesting glimpse into the origins of the story's characters. Scar once belonged to a royal pride, but Mufasa enters the scene owing to a flood that lands him with this group whose leader deems him an outsider.
Scar was actually named Taka when he first encountered Mufasa. It wasn't until things began going south that prompted his non-biological brother to call him Scar. While we know him as a series antagonist, his scar was a result of him saving Mufasa from the Outsiders, a pride of banished lions in the story.
Mufasa: The Lion King introduces Kiros, who gave Taka the scar
Now that Mufasa becomes a part of the new pride after the flood washes him away from his land, he slowly adjusts to the lion clan. But it isn't the clan's destiny to survive for long as the Outsiders kill everyone except Mufasa and Taka in Mufasa: The Lion King.
The leader of the Outsiders is Kiros, who then pursues the brothers but fails to catch them initially. During their journey following the death of their clan, they meet a younger Rafiki, Zazu, and Sarabi. Sarabi and Mufasa begin a romantic affair, of which Taka gets jealous, leading him to betray them.
Taka meets Kiros and proposes that he kill his non-biological sibling. Kiros' son is killed by Masego, Mufasa's biological father, which gives him the motivation to take revenge.
A battle follows between the Outsiders and the animals, where, in a particular fight, Kiros dominates Mufasa and corners him in a cave. Now, he has an upper hand and can easily kill the son of his enemy. Blood for blood.
Taka watches this and has a sudden change of heart, and when Kiros attacks Mufasa, who is lying down, Taka intervenes. Instead of Mufasa, Kiros accidentally hit his brother, a hit that gave him a scar on his eye.
Taka has more scars than we can see
It all began like Taka and Mufasa would grow to become two happy brothers, but the former realizes it was wrong for him to bring a lion from a different pride. His father, Obasi, never welcomed Mufasa into the clan. Later, Taka became somewhat of an embarrassment in his father's eyes.
He holds romantic feelings for Sarabi, who later becomes the Queen of Mufasa. After leaving his non-biological brother to die, he can be seen making advances on Sarabi, which never work out.
After Kiros is killed in Mufasa: The Lion King, both the brothers come face to face again, and Mufasa tells him that he will never forgive the betrayal. This is when he vows to address Taka as Scar.
He could've been the Loki of Mufasa: The Lion King's story had he spared his brother. At least the God of Mischief didn't do it with his noon-biological brother, though he tried.