Maybe Dean gets cursed by a witch. The tail could be a side - effect of the curse that slowly changes him. He has to find a way to break the curse while still hunting monsters, and Sam has to deal with his brother's new 'accessory' and the problems it causes during their hunts.
There probably is. The Supernatural fandom is quite large and creative. Some fans might have come up with the idea of Dean having a tail for various reasons, perhaps as part of a magical or cursed storyline. It's worth exploring the major fanfiction hubs to find it.
In this 'dean becomes a dragon supernatural fanfiction', it could start with Dean being in a fight with a powerful demon. During the battle, the demon casts a spell that backfires and instead of killing Dean, it turns him into a dragon. Dean then has to deal with the fact that he is no longer fully human. He may have to hide from other hunters who might see him as a threat. Meanwhile, he tries to figure out how to use his new dragon abilities to continue his fight against the supernatural, all while trying to find a way to turn back to his normal self.
She could be a nephilim, half - angel and half - human. This would put her in great danger as both angels and demons would be after her. Sam and Dean would have to constantly protect her and figure out a way to keep her powers in check.
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.
Well, perhaps the baby is a key to closing a major portal to the underworld. Dean discovers this and has to raise the baby while also preparing it for this great task. Along the way, he gets help from Sam and other friends. They face various challenges from demons who want to stop the baby from fulfilling its destiny.
Perhaps he's on a self - imposed exile. Dean has always been the type to take on burdens himself. He could have discovered something about himself, like a dark prophecy related to him endangering Sam or his friends, and decided to disappear to keep them safe.
Perhaps in this fanfiction, Harry is born with a tail due to some ancient magic that was dormant in his family line. This tail could be a sign of a special power he has yet to discover. He might have to go on a journey to find out the origin and purpose of his tail, with his friends Ron and Hermione helping him along the way. They could explore hidden chambers in Hogwarts or ancient wizarding libraries to uncover the mystery.
Maybe in the fanfiction, Hermione discovers that her tail is a sign of a long - lost magical heritage. This could lead her on a journey to discover more about her family's past and the powers associated with having a tail. Along the way, she might face challenges from those who want to use her new - found connection for their own gain.
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.
Perhaps the cat tail is a curse at first. Adrien has to deal with all the awkwardness it brings, like not being able to hide it properly. But as time goes on, he realizes that the tail gives him an edge in certain sports or activities, like climbing. So it turns from a curse to a sort of advantage.