Perhaps the hellhound has a special power like being able to travel through fire unharmed. This power could play a crucial role in the story. For example, the main character and their friends get trapped in a burning building, and the hellhound uses its power to create a path through the fire, allowing them to escape. The fanfiction could then focus on how they keep this power a secret from the school authorities.
One possible scenario in a Harry Potter pet hellhound fanfiction could be that a student secretly adopts a hellhound puppy. They manage to keep it hidden in the Room of Requirement. The hellhound grows up with a special bond to the student, and it eventually helps them during a dangerous encounter in the Forbidden Forest, using its fierce loyalty and powerful abilities.
Well, they might go on adventures like exploring the Forbidden Forest together. The hellhound could use its keen senses to find hidden paths or magical creatures. Harry could rely on the hellhound's strength and loyalty during dangerous situations, such as when facing dark wizards or magical beasts that lurk in the forest.
It could be that the hellhound was a gift from a mysterious wizard. This wizard recognized Harry's need for a powerful and loyal companion, so he gave him the hellhound. The hellhound might have some special powers or abilities that the wizard thought would be useful to Harry in his future adventures.
Well, he might sell magical creatures like owls, toads, and perhaps even some rare and exotic ones like hippogriffs. He could use his knowledge of magic to take care of the pets and also teach the customers how to handle them properly.
One possibility is that it could communicate with the characters in a different way. If it was a cat like Crookshanks, it could use its human form to convey important information that it couldn't as a cat. Maybe it has seen something in the wizarding world that only an Animagus could understand and needs to tell Hermione and her friends.
The plot could involve the hellhound helping Dean in battles. It could use its strength and speed to take on enemies. For instance, when facing a group of demons, the hellhound could be the first to attack and give Dean an advantage.
In some fanfictions, Harry Potter might discover a hidden magical object in the bathroom. For example, a secret passage could be revealed behind a mirror or a sink when he's in there alone, leading him to a new adventure or a place filled with ancient magic.
Well, Harry could get involved in some really off - the - wall pranks. Since he's bored, he might decide to play some elaborate tricks on his friends, like magically changing their voices during class.
In fanfiction, if Harry Potter disappears, it could lead to chaos at Hogwarts. His friends like Hermione and Ron would likely be on a desperate search for him. They might suspect the Dark forces at work again, and start investigating all the possible places where Harry could have been taken or hidden.
In fanfiction, Harry Potter could be wrongly accused of using dark magic in court. Maybe some false evidence is presented against him, like a planted dark magic item. And he has to fight to prove his innocence.