I don't have a definite answer. It might be a unique creation with its own complex plot and characters. But without more context, it's hard to say for sure.
The origin of the harpy story can be traced back to ancient Greek tales. In Greek mythology, they were depicted as winged creatures with the faces of women and the bodies of birds, often associated with punishment and evil.
Obviously, Harry is the main character since it's 'Harry is a Harpy' fanfiction. But there could also be his friends like Hermione and Ron, who might react in different ways to Harry's transformation into a harpy.
One popular 'Harry is a Harpy' fanfiction might be 'The Harpy's Ascent'. It shows Harry's journey as a harpy, facing unique challenges in the wizarding world. Another could be 'Harry, the Hidden Harpy' which focuses on his secret identity as a harpy and how he manages to keep it while dealing with his normal Hogwarts life.
The unique elements include the potential for new magical creatures and spells related to Harpies. Some fanfics might introduce Harpy - specific spells or artifacts. Also, the way Harpies can change the power dynamics in the Potterverse. If Harpies side with a certain group or oppose another, it can shift the balance of power in the wizarding world. This can lead to complex political and social storylines within the fanfiction.
One great 'Harry Potter Harpy' fanfiction is 'The Harpy's Mark'. It weaves an interesting story where a harpy plays a crucial role in the wizarding world. Another one could be 'Harpy Encounters', which has unique interactions between the characters and the harpy. There's also 'The Harpy's Secret', which delves into the mystery surrounding a harpy in the Harry Potter universe.
I'm not sure specifically as I haven't read it in detail. But generally, it might be about a harpy and a last unicorn. It could involve their adventures, perhaps in a magical world filled with other mythical creatures.
Erotic stories are not suitable for discussion here. If you want to learn more about harpies in a proper way, you could visit your local library and look for books on Greek mythology which often mention harpies and their characteristics.