It might be that there is no traditional 'father' in the sense we know. Maybe King Ghidorah's pregnancy was a result of some sort of self - replication ability that it has, which was activated by a unique event in the Monsterverse. So it's a completely new form of reproduction for it.
In the world of fanfiction, it could be another powerful kaiju. Maybe a new, mysterious kaiju that was introduced into the Monsterverse secretly. This new kaiju could have some special abilities that were passed on to King Ghidorah's offspring.
One possible plot could be about how the other kaiju react to King Ghidorah's pregnancy. Maybe Godzilla would be initially shocked and then start to consider if this new situation changes the power balance among them.
If it's a crossover with the Power Rangers, the Rangers themselves would be obvious choices. They could combine their Zords and powers to take on King Ghidorah. Each Ranger could use their unique skills and weapons to try and damage the three - headed monster.
It could be her long - time love interest in the original story. Maybe they had a moment of passion that led to the pregnancy.
Well, perhaps Harry discovers he has some hidden power that is related to King Ghidorah. Maybe he can transform into King Ghidorah - like form in times of great danger at Hogwarts, using his new - found abilities to protect the school from dark wizards. The other characters would be both amazed and a bit scared of this new Harry.
Perhaps it's a human boy who accidentally wandered into the fairy world. Tinkerbell, being curious, interacted with him, and in a magical turn of events, she became pregnant. This would create a lot of drama as the fairy world and the human world don't usually mix in such an intimate way, and there would be a struggle to figure out how to handle this situation.
It could be Tony. Their relationship has always had a certain chemistry, and it would make sense for him to be the father. It would add a new layer to their already complex dynamic.
It could be Asuma. Since they had a relationship in the original series, it would be a logical choice. Their relationship was a significant part of the story, and having him as the father would add depth to the fanfiction, with the story exploring their future together as parents.
There's also the possibility that it was an accident involving a time - traveling character. Perhaps someone from the future came back in time and accidentally caused Frieza's pregnancy through some unforeseen interaction. This would open up a whole can of worms in terms of time - travel paradoxes and how it affects the future of the 'Dragon Ball' universe.
It could be one of her close friends or classmates in 90210. Maybe someone she had a brief fling with during a party or a school event.
It could be Finn. They have a strong bond in the movies, and it would be an interesting dynamic to explore their relationship as they prepare for a child together.