Godzilla himself would obviously be a main character. His actions and reactions would drive much of the story. Another possible main character could be Pinkie Pie. Her unique view of the world and her ability to make friends with anyone could play a role in how the ponies deal with Godzilla. Maybe she throws a party for Godzilla to make him feel welcome.
Riku could also play a major role. His more complex and sometimes brooding personality could add an interesting dynamic. He might start off as a bit of an outsider in Equestria but eventually come to understand and care for the ponies, especially if there are some ponies who remind him of his friends from his own world.
For a main character from the human side, it's Michael Rosen. And from MLP, I think Rarity could be a main character. Rarity would be fascinated by Michael Rosen's different style of dress (compared to ponies) and might try to create pony - versions of his outfits. And Applejack could also be a main character, showing Michael Rosen how to work on the farm and sharing her values of hard work and honesty with him.
Link and Twilight Sparkle would be obvious main characters. Link with his skills in combat and exploration, and Twilight Sparkle with her powerful magic and intelligence. They could form an unlikely duo to face challenges.
King Kong would be an obvious choice. The epic battles between Godzilla and King Kong are well - known, and a fanfiction could explore different scenarios of their encounters, like what if they had to team up against a greater threat.
Twilight Sparkle could be a main character. Since she is very intelligent and curious, she might be the one who gets drawn into the Creepypasta - related events. Her knowledge of magic could also play a crucial role in either solving the problems caused by the crossover or in getting trapped in the first place.
In addition to the main trio from Harry Potter, Dumbledore could also be a major character. His wisdom and powerful magic could be crucial in dealing with Godzilla. He might be the one to come up with a plan to either communicate with Godzilla or find a way to send it back to its own world. On Godzilla's side, there could be some sort of a magical connection or entity that is controlling it or has a link to it, and that could also be considered a main character in the fanfiction.
Definitely Dr. Alan Grant. He has so much knowledge about dinosaurs that he could be the key in trying to understand how Godzilla affects the dinosaurs in the park. Also, Godzilla himself would be a main character in a sense. His actions and presence would drive the story. And maybe Claire Dearing, as she has experience managing the park and could be involved in the efforts to either protect the park from Godzilla or figure out how to co - exist with him.
King Kong is definitely one of the most popular. Their long - standing rivalry in the monster movie genre makes it a natural choice for fanfiction crossovers. Another popular one is Mothra. The relationship between Godzilla and Mothra in the original movies can be explored further or twisted in fanfiction crossovers. Also, characters from the Ultraman series are often crossed over with Godzilla. The different powers and abilities of Ultraman characters create interesting dynamics when pitted against Godzilla.
One idea could be that Godzilla somehow ends up in Equestria. The ponies are initially terrified but then realize Godzilla is more interested in protecting the land from some new dark force that has emerged there. Maybe Godzilla's size and power are a mystery to the ponies at first, and Twilight Sparkle tries to study him to figure out how he got there.
Definitely the Eva pilots, Shinji, Asuka, and Rei. They are the main characters in Evangelion and would be at the forefront of dealing with Godzilla's appearance in their world.