Sasuke is a likely candidate in some fanfictions. Given their complex relationship throughout the original series, it's easy to imagine a scenario where Sasuke's thirst for power or some misinformation leads him to betray Naruto. Another could be Sakura. If she becomes jealous of Naruto's growing power or if she's manipulated by outside forces, she might turn against him.
It could be Zabuza. He might pretend to be on Naruto's side at first during the Wave events and then betray him when Naruto least expects it. Zabuza is a mercenary and his loyalty can be easily swayed.
It could be one of Naruto's former teammates from a mission. Maybe this teammate has some hidden motives or has been bribed by an enemy. They could spread false information about Naruto within the community, making it seem as if Naruto has betrayed them all during a crucial mission. This would cause the rest of the community to lose trust in Naruto.
He could betray Naruto to Orochimaru. Orochimaru is always looking for powerful allies and new test subjects. Lucifuge might be lured by Orochimaru's promises of power and knowledge. Orochimaru could use Lucifuge to get closer to Naruto in order to steal the Nine - Tails' chakra or some other valuable asset.
Even Hinata could potentially be written as a betrayer in some really out - there fanfictions. Maybe in a crossover where there are mind - control elements, Hinata could be forced to turn against Naruto. But this would be a very different take on her character as she's usually portrayed as extremely loyal to Naruto.
He might be mind - controlled. Some dark magic or an evil character could take over Naruto's mind and force him to act against the ponies in the MLP world.
There's a chance that some of his former teammates who have their own insecurities. For example, someone who always felt in Naruto's shadow might be the one to betray him in order to get more recognition for themselves.
Sasuke could be a likely candidate. He's always been somewhat jealous of Naruto's growth and popularity. In the stress of the Forest of Death, he might let that jealousy get the better of him and betray Naruto. Maybe he thinks that by getting rid of Naruto, he can be the only strong one in the team.
I recommend Naruto: Hegemony of the Heroes and Single-player Muye to you. These novels were all related to the world of Naruto, and the plot of Naruto crossing over and betraying Koba was also reflected in the story. The plots and characters of these novels were very interesting and worth reading. I hope you like this fairy's recommendation. Muah ~😗