Another possibility is a new, original character created for the fanfiction. This character could be jealous of Naruto's power, even if it makes him feel like a monster. The new antagonist could try to steal Naruto's power or make him feel even more like an outcast, all while Naruto is expressing his feelings through the song 'I Feel Like a Monster'.
It could be Orochimaru. He might see Naruto's 'monster - like' state as an opportunity to use him for his own experiments. Orochimaru has always been interested in power and Naruto's new, strange form could be a source of great power that he wants to harness for his evil plans.
The main antagonist could be a demon who is threatened by Naruto's singing ability. This demon might be using people's inner demons to gain power, and Naruto's singing that calms those demons is ruining its plans.
In this fanfiction, the main antagonist might not be a person at all. It could be a situation or a force. For example, a curse that affects Jasper's singing ability when he's around Bella. This would be a unique twist, as he has to overcome this obstacle to continue singing for her and winning her heart.
Perhaps in the fanfiction, Naruto is under a genjutsu that makes him see himself as a monster. When he sings 'I Feel Like a Monster', it's a cry for help. His friends start to notice his strange behavior and try to break the genjutsu. Along the way, Naruto's singing might draw the attention of an enemy who wants to use his 'monster - like' state for their own evil purposes.
It could be Orochimaru. He's always interested in new forms of power. In this fanfic, he might discover the Monster Rancher world and try to use the monsters there for his evil experiments and plans. He could be trying to create some sort of ultimate monster by combining the powers of the Naruto universe and the Monster Rancher monsters.
It could be Orochimaru. He has always been interested in experimenting with new forms and powers. If Naruto becomes a monster, Orochimaru might see him as a perfect specimen to study or use for his own evil plans.
He could be singing for Hinata. After all, she has been by his side through a lot. It would be a very romantic gesture, showing his love and appreciation for her.
The main antagonist could be the one who turned Percy into a monster in the first place. Maybe it's a vengeful god who feels Percy has wronged them in some way and decides to punish him by turning him into a fearsome beast.
One possible plot could be Naruto using the song as a form of inspiration during a tough battle. Maybe he's facing an enemy that seems unbeatable, and the lyrics of 'I Will Not Bow' give him the mental strength to keep fighting. He might start singing it quietly to himself at first, and as his resolve grows, his voice gets louder, inspiring his comrades as well.
Maybe a new character could be introduced as the main antagonist. This character could be from a hidden village that has been secretly observing Naruto's growth. They realize his potential as a weapon and plan to steal his power or use him to start a war against other villages.
Krampus himself could be the main antagonist. Since he is a figure known for punishing the naughty, his actions in the Naruto world might be seen as unjust. He could be going after ninjas without really understanding their motives, causing a lot of trouble for the main characters.