Fluttershy would be heartbroken. She would likely retreat to her cottage for a while, surrounded by her animal friends. But she would also be the one to first start thinking about how to honor Twilight's memory. Maybe she would start a garden in Twilight's name or try to teach the other ponies about the gentle and kind ways that Twilight had. The other characters would look to Fluttershy for comfort as she has a very calming presence, even in her own grief.
Some characters might be amused. They could enjoy seeing how the fans see them and their world. For instance, Pinkie Pie would probably find the fun and wacky fanfictions really entertaining.
On the other hand, there are fans who appreciate the darker and more complex storylines that these types of fanfictions can offer. It shows a different side of the characters and how they would cope in difficult situations. They might see it as a way to explore deeper emotions and themes within the My Little Pony universe.
Well, the pony's closest friends would initially be startled but then they'd start to look for solutions. They'd likely research in old books or ask around if anyone has heard of such a transformation before. As for the ponies who were not so close to the transformed pony, they could either be indifferent or use it as an excuse to spread rumors. Some might say that the pony must have done something bad to deserve such a transformation.
Twilight Sparkle would be analyzing the action scenes. She'd be trying to understand how the chakra system works in the fanfiction and how it powers the amazing jutsu. She might even try to compare it to her own knowledge of magic. Pinkie Pie would be bouncing around, adding her own running commentary to the action, making it more like a party - like event.
Some characters might be confused. Pinkie Pie, for example, might not fully understand what's going on at first. But then she would probably turn it into a fun adventure, trying to help the regressed pony have a good time while also trying to figure out how to reverse the situation. She could throw a 'get - better - soon' party for the pony.
One possibility could be that Twilight sacrifices herself to save Equestria from a powerful dark magic. Maybe she uses all her magic power to seal away an ancient evil, and in the process, it drains her life force completely. This would lead to a very sad and solemn atmosphere in Equestria, with her friends left to mourn and remember her great deeds.
Bakugo would be deeply affected. He might initially deny his feelings, but then would be filled with guilt for all the times he was mean to Deku. He could become more withdrawn and focused on becoming a hero worthy of Deku's sacrifice.
Sakura would be heartbroken. She would probably blame herself for not being able to protect Naruto. She might also start to train harder to fill the void left by Naruto.
Sakura would be devastated. She has been Naruto's teammate for a long time. She might blame herself for not being able to save Naruto. She would probably become more withdrawn and focus all her energy on finding a way to bring Naruto back, even if it's impossible. Kakashi would be deeply saddened too. He has always seen Naruto as a special student. He would likely feel that he has failed as a teacher.
Their relationship could start off as one of pure enmity. Twilight would see King Sombra as a great threat to Equestria. But as the story progresses, if they were forced to interact more, Twilight might start to see some small glimmers of humanity (or pony - ity) in Sombra. For example, she might notice that he has a love for a certain kind of magic that is not entirely evil.