Erik may initially be in disbelief. He could think it's some kind of trick or misunderstanding. He might start investigating, trying to find out if it's really true. He could look into Peter's past, talk to people who knew Peter's mother, etc., to confirm the truth of this revelation.
In the fanfiction, Erik could also be in a state of shock at first. He might struggle to accept this new reality. After all, having a son is a huge thing, especially when he had no idea before. He could be hesitant to approach Peter, worried about how Peter will react to the news.
He would be very angry. Naruto is not one to take betrayal lightly.
He might be shocked. Fanfiction can often be quite different from the original source material, and the nogitsune concept is already pretty out - there. So the shock value could be high.
In some fanfictions, Vegeta is initially shocked. He might stand there in silence for a moment, his eyes wide. Then he could feel a strange mix of emotions, like pride and fear. He's proud to be having a child with Bulma but also fears for the safety of his family given his dangerous past and the many enemies he has.
Loki could be surprisingly empathetic. He would see a bit of himself in Peter, perhaps, as someone who has been misunderstood. So he'd try to befriend Peter and give him advice on how to handle the bullies without using violence, like using his intelligence.
In fanfiction, when Tony finds out Peter is his son, their relationship often becomes more complex. At first, Tony might be in shock. He could start to be overly protective of Peter, using his resources to keep him safe. This new knowledge also deepens their emotional bond. Tony may start to see Peter not just as a protege but as his own flesh and blood, leading to more personal and heartfelt conversations between them.
In other cases, Teela's reaction could be a mix of emotions. She could be happy that Adam is such a powerful hero but also a bit sad that he felt the need to keep it a secret from her. This discovery might make her question their entire relationship up to that point, and she may need time to sort out her feelings about it all.
He would likely be initially shocked but then filled with joy. He might become very protective, using his skills to keep the pregnant person safe from any harm in the dangerous Middle - earth setting. He could also start making plans for the future, like finding a safe place for the family to live.
In many fanfictions, Naruto's reaction is a mix of shock and determination. The shock comes from finally learning about his clan, and the determination is to uphold the Uzumaki name. He could start wearing something that represents the Uzumaki clan, like a symbol on his clothing. He also may become more protective of the Uzumaki secrets and legacies he discovers.
Steve is usually shocked and conflicted. He might question a lot of things and have mixed emotions.