Perhaps it's a curse or a spell gone wrong. In the world of PJO, there are many magical mishaps. It could be that someone intended to harm Percy and this was the result. Or it could be a side - effect of a powerful artifact he came into contact with.
It might also be a symbol of his evolution as a character. As Percy grows and faces new challenges, the tail could represent a physical manifestation of his changing nature. It could be a way for him to access new abilities or areas that were previously inaccessible to him, like deeper parts of the ocean or secret underwater realms.
It could be a group of fans who collaborate on the story. They might each contribute different parts or ideas. For example, one person could be good at writing action scenes, another at character development, and they come together to create this 'Percy has a tail' fanfiction.
One common theme is adventure. Percy is always going on some sort of journey or quest. Another is friendship, as he relies on his friends like Annabeth and Grover. And of course, there's the theme of power, especially related to Percy's unique abilities which might be emphasized more in fanfics about his tail.
One popular Percy Tail PJO fanfic is 'The Lost Hero's Quest'. It takes the characters on a new adventure filled with mystery and danger. Another great one could be 'Percy and the Underworld Revisited' which delves deeper into the relationship between Percy and the Underworld. There's also 'Tail of the Titans' that has an interesting take on the battles against the Titans from Percy's perspective.
Well, perhaps Percy is a character in a magical world. He could have been cursed or exposed to some strange magic that causes him to grow a tail. Maybe it starts as a small, furry growth that he tries to hide, but as the story progresses, the tail takes on a life of its own and gives him unique abilities like climbing better or sensing danger.
A really interesting fanfic in this genre is 'Fairy Tail's Demigod Adventure'. It delves deep into how the powers from Percy Jackson's world would interact with the magic in Fairy Tail. For example, how a demigod's strength would fare against a Fairy Tail wizard's spell. It has a great plot that keeps you hooked from start to finish.
First, you need to understand the two universes well. Know the rules of magic in Fairy Tail and the nature of demigods in Percy Jackson. Then, create a unique plot. Maybe a crossover event that brings the two worlds together in an unexpected way, like a powerful dark force that threatens both.
His character could change in several ways. He would have a new sense of identity. No longer just a demigod with powers, but now something more 'aquatic'. He might be more curious about his father's world and want to spend more time there. This could lead to some conflict with his human side, as he has to balance his life on land and his new underwater existence. Also, he could become more of a leader among the sea creatures, as he has a unique connection to them through the tail.
Obviously, Percy would be the main character. But there could also be his best friend, let's say Jack, who helps him through the whole ordeal of growing a tail. Maybe there's a wise old mentor figure, like an old wizard named Merlin, who has some knowledge about such strange transformations.
The plot could involve Percy waking up one day and finding a tail instead of his legs. At first, he's confused and a bit scared. But then he realizes it gives him some new abilities, like super - fast swimming. He could then go on a quest where his tail is both an advantage and a hindrance. For example, it might be hard to move on land, but in water, he can out - swim any monster.
Well, perhaps the plot could involve Percy being given the tail as a sign of special favor from Poseidon. He might then have to learn how to use the tail in underwater adventures, like swimming faster or having better control in the ocean currents. It could also lead to him being able to communicate more effectively with sea creatures, since he now has a more 'aquatic' form.