Most likely Harry Potter himself would be a main character since he is in the title as the 'king of dark creatures'.
There could also be some dark creatures that are important characters. For instance, a powerful werewolf or a vampire who either support Harry in his new role or oppose him. These dark creatures would add an interesting dynamic to the story as Harry has to interact with them on a different level now that he is their 'king'.
In addition to Harry, his friends like Hermione and Ron could also be main characters. They would probably play important roles in Harry's journey as the king of dark creatures. Maybe they would be his advisors or the ones who help him face the challenges that come with this new role.
It could be about Harry Potter having some sort of connection or role as the 'king' of dark creatures in a fan - created story. Maybe he discovers a hidden power within himself that gives him control over these usually feared beings.
Werewolves are also a popular dark creature in fanfiction. Writers may explore their relationships with other characters in more depth, like how they interact with Hogwarts students. For example, a werewolf character could be a misunderstood outcast trying to find his place.
It could also be an existing character from the Harry Potter series like Hagrid. Given his love for all creatures, including some that are considered dangerous, he could be at the center of a story about the 'Lord of Dark Creatures'. He might be trying to protect these creatures from those who would use them for evil purposes or trying to rehabilitate them.
Perhaps vampires. Vampires are often powerful and if they have a reason to protect Harry, they could be very effective. Maybe they have a past connection with Harry's family or they see a future benefit in keeping him alive. There could also be creatures like basilisks. If they are tamed or controlled in the fanfiction, their deadly stare could be used against Harry's enemies instead of against him.
The Acromantula. Harry could come across a young Acromantula that is different from the rest of its kind. Instead of being aggressive, it shows an interest in learning about the human world. Harry, being the kind - hearted person he is, decides to befriend it. He might even teach it some basic spells or communicate with it using Parseltongue.
In Harry Potter fanfiction, the Boggarts can be very interesting. Writers might create unique forms that the Boggarts can take, based on the fears of different characters. The Thestrals are also popular. Fanfics could explore their connection to death in more depth or have them play a more significant role in the wizarding world. Additionally, the Nundu is a dark creature that might be given more detailed descriptions in fanfiction, like its venom having even more deadly effects.
The Dementors are quite popular. Their ability to suck out happiness and their overall spooky nature make them fascinating to write about. Werewolves are also common, especially with the whole Remus Lupin storyline in the original books. And the Basilisk is often used in fanfiction, with its deadly stare and connection to Salazar Slytherin.
In fanfiction, dark creatures can be more sympathetic. For instance, a vampire might be shown as a victim of his own condition rather than just a menacing figure. This gives readers a new perspective on these usually feared beings.
There could also be a group of pure - blood wizards who look down on Harry's connection with magical creatures. They see it as beneath them and try to undermine his position as the king of magical creatures. They might spread false rumors about him and try to turn other wizards against him.
Dark creatures in Harry Potter fanfic can serve as antagonists. They create conflict and danger for the main characters. For instance, a group of Acromantulas could attack the Hogwarts Express, leading to a thrilling chase and battle.