Well, perhaps a thestral. Since Harry can see them, in this fanfiction they might be attracted to him in a rather odd way. They could fly around him in formation like a circus act whenever he's outside. And then there's the dementors. But instead of being all gloomy and soul - sucking, they might be attracted to Harry because he has some kind of 'bright' aura that they find really confusing and they just keep hovering around him trying to figure it out.
He might become more laid - back. Since he's constantly dealing with these dark creatures in a humorous way, he doesn't take things as seriously. For example, instead of being scared of a werewolf, he might just joke about its hairy appearance.
Some dark creatures could have magical powers that are not fully understood. They could create barriers or illusions to hide Harry from his pursuers. In the fanfiction, they may also use their knowledge of the dark arts in a positive way to counter the dark magic aimed at Harry. They might be able to sense danger before it arrives and take preemptive action to keep Harry safe.
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.
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.
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.