In a fanfiction, the hellhound could be a protector of Percy instead of an enemy. It could be that the hellhound was once a pet of a powerful god who turned evil, and Percy helps it break free from the evil influence. So the hellhound then becomes loyal to Percy and helps him in his quests, like warning him of approaching monsters or even fighting alongside him against stronger foes.
It could be a powerful sorcerer who wants to control the Hellhound King for his own evil purposes. This sorcerer might be trying to use the Hellhound King's strength to take over Camp Half - Blood or other important places in the Percy Jackson world.
There's also a fanfic where a hellhound becomes a protector of Percy's sister. In this 'Percy Jackson Hellhound Fanfiction', the hellhound's loyalty and the way it interacts with the characters in the Percy Jackson universe are really interesting. It shows a different side of these usually menacing beasts.
Fanfiction.net is a great place to start. There are tons of 'Percy Jackson Hellhound Fanfiction' there. You can search by tags like 'hellhound' and 'Percy Jackson' to narrow down the results.
Well, perhaps in the fanfiction, the Hellhound King has a connection to an ancient prophecy. Percy is tasked with either killing it or taming it to fulfill the prophecy. As Percy delves deeper, he realizes that the Hellhound King has a complex personality and motives that are not just about destruction. It could also be that the Hellhound King has a hidden lair full of secrets and treasures that Percy needs to explore for some reason, like to find a weapon to defeat a new threat.
Perhaps the hellhound is a part of a prophecy. In the fanfiction, it could be foretold that a hellhound will either bring great destruction or be the savior of the demigod world. Percy then has to figure out which path the hellhound is on and try to influence it to the side of good if it's the one that could cause destruction.
The hellhound is also a key character. Depending on how the fanfiction is written, it could be a villain at first and then turn into an ally. It might have unique abilities and a backstory that makes it an interesting addition to the story.
One idea could be the hellhound having a secret bond with Percy. Maybe it starts out as a dangerous encounter but as Percy tries to understand the hellhound's motives, they form an unexpected alliance. This could lead to them facing off against common enemies in the Percy Jackson universe.
One common theme is identity. Percy has to figure out who he is now as a hellhound, different from his normal self. Another theme could be acceptance, as he has to deal with how others perceive him in this new form.
In some Percy Jackson fanfictions, he might be depicted as the King of Hell, but this is a departure from the original series. Fanfic authors like to take creative liberties and explore different scenarios for their characters.