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.
A Dementor could be an interesting choice. In the fanfiction, it could be that there is a Dementor that is not fully corrupted by the dark side. Harry, with his ability to produce a powerful Patronus, catches the attention of this Dementor. Through a series of events, he manages to befriend it and discovers that there is more to Dementors than just being soul - sucking creatures.
One possible plot could be that Harry discovers a misunderstood dark creature, like a werewolf cub that has been separated from its pack. He decides to befriend it and protect it from those who would harm it just because of what it is. As he befriends the cub, he learns about its unique abilities and how it views the wizarding world.
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.
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.
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.
Most likely Harry Potter himself would be a main character since he is in the title as the 'king of dark creatures'.
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.
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.
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.
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.
One interesting creature would be the phoenix. It's a symbol of rebirth and has amazing powers. Living with a phoenix, Harry could learn a great deal about life, death, and renewal. The centaurs are also fascinating. If Harry were to live with them, he could gain wisdom about astronomy, divination, and the secrets of the forest. And don't forget the merpeople. Living with them underwater would be a whole new world for Harry, full of unique magic and challenges.