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.
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.
In Harry Potter Dark Arts fanfiction, Voldemort is often a very popular character. His power and his connection to the Dark Arts make him a central figure in many stories. Another popular character is Severus Snape. His complex relationship with the Dark Arts and his dual nature of being both a potential ally of the Dark side and a hero on the side of good add a lot of depth to fanfictions. Bellatrix Lestrange is also popular as she is a fanatical follower of the Dark Arts.
Harry Potter himself is often a popular character in these fanfictions. His connection to Voldemort and his experience with the dark arts throughout the original series make him a natural choice for further exploration in fan - created stories. Writers can put him in different scenarios involving the dark arts, like facing new dark wizards or dealing with ancient dark arts relics.
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.
Most likely Harry Potter himself would be a main character since he is in the title as the 'king of dark creatures'.
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.
Voldemort is often still a very popular dark mage in these fanfictions. Even though he was defeated in the original series, in fanfictions, he can be resurrected or his influence can still be felt in different ways. Another popular one is often a new, original dark mage created by the fanfiction author. This character can bring fresh and unique storylines. There's also Bellatrix Lestrange who, in some fanfictions, is given more of a central role as a dark mage and her powers and motives are explored further.
Hagrid is often a central character in 'Harry Potter breeding creatures fanfiction' as he has a great love for magical creatures. He might be involved in breeding projects, using his extensive knowledge. Another popular character could be Newt Scamander. Given his expertise in magical beasts, he could be a mentor or even directly involved in breeding rare or endangered creatures in fanfiction stories.
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.