One common theme is self - discovery. Harry often has to figure out what it means to be a merman and how to use his new abilities. Another theme is acceptance, both from himself and from others. Since he's different now, he has to face those who might not accept him at first.
A power - hungry merman who wants to rule over all the mer - people. He sees Harry as a threat because Harry has some special powers or a claim to the throne that he doesn't want to share.
One great merman Harry Potter fanfiction is 'The Merman's Magic'. It has an interesting plot where Harry discovers he has a connection to the merman world. The author does a great job of blending the magical elements of the Potterverse with the unique lore of mermaids and mermen.
One possible plotline could be Harry trying to hide his merman identity while at Hogwarts. He might face challenges in the magical underwater areas of the school, like the lake. Another could be that he discovers his merman heritage and has to go on a journey to learn more about it and his new powers.
Maybe Harry is the only merman in a world where mermaids are common. He faces discrimination at first but then uses his unique abilities to save the underwater kingdom from a great danger. For example, he might have a special power to purify poisoned waters that the mermaids don't have.
There's also 'Harry, the Merman Prince'. In this story, Harry is a merman prince who is on a quest to find a magical item to save his underwater kingdom. It's full of action and adventure, with some great character development for Harry. The relationships he forms with other underwater creatures are really engaging.
The combination of two very different concepts. Harry Potter is all about magic on land, in a school, with wizards and witches. Mermen bring in a whole new element of an underwater world full of mystery and different kinds of magic. It's like opening a new door in the already vast Potterverse.
Well, it could start with Harry being cursed by a dark spell during one of his adventures near the Black Lake. Suddenly, he starts to feel strange sensations and discovers his legs are fusing together and turning into a merman's tail. He has to adapt to this new form. Maybe he discovers he can communicate with the merfolk in a different way now and they have some secrets about the wizarding world that only he can learn in this form.
It could be a rogue wizard who wants to use Harry's new merman form for his own evil purposes. Maybe he wants to study Harry's transformation to find a way to create an army of half - human, half - merman creatures to take over the wizarding world and the underwater realm.
Yes, there are some fanfictions with the concept of Harry Potter as a merman. You can try searching on popular fanfiction websites like FanFiction.net or Archive of Our Own. Just type in relevant keywords and you might find some interesting ones.