I'm not sure which specific fanfic you're referring to. There could be many fanfictions that involve Harry being an Animagus and having some sort of connection with a chair and a unicorn. You might want to check some popular fanfic websites like Archive of Our Own or FanFiction.net and search for relevant tags like 'Harry Potter', 'Animagus', 'Chair', 'Unicorn' to find it.
Well, if it's a fanfic about Harry as an Animagus in relation to a chair and a unicorn, it could be really creative. Maybe Harry's Animagus form has some special interaction with the chair that is enchanted in some way, and the unicorn might be a symbol of magic or a helper in the story. However, without more details, it's hard to say exactly what the fanfic is about. It could also be a humorous take where the Animagus form has to deal with the chair as an obstacle and the unicorn is a magical guide through this strange situation.
One possible fanfic could be about Harry and Remus exploring a magical forest where they encounter a unicorn. The unicorn has a special connection to a magical animagus who can transform into a chair. Maybe the animagus in chair form holds a key to a hidden mystery in the forest that Harry and Remus need to solve to protect the unicorn and the forest's magic.
A fanfic might be centered around a prophecy that involves Harry, Remus, a magical animagus in chair form and a unicorn. The prophecy states that only when these four elements come together can a great evil be defeated. Harry and Remus start searching for the animagus - chair and the unicorn. Once they find them, they have to learn to trust each other and combine their powers. The unicorn's pure magic, the animagus - chair's unique abilities, Harry's courage and Remus' wisdom will be the keys to victory against the looming evil.
I haven't come across such a specific fanfic myself. You could try asking in Harry Potter fanfic communities on social media platforms like Reddit. There are subreddits dedicated to Harry Potter fanfiction where fans are really helpful in finding or even creating fanfics with specific elements.
A possible fanfic could be about Harry and Remus in a magical forest. They find a chair made of unicorn horns (not in a cruel way, perhaps it was a gift from the unicorns). A wolf approaches them, but it turns out to be a friendly werewolf under Remus' influence. Harry and Remus have a deep conversation while sitting on the chair, and the unicorn comes to join them, symbolizing the unity of different magical beings in their world.
Perhaps a story about an adult wizard who was cursed and turned into a unicorn animagus against his will. He then has to navigate the wizarding world in this new form, facing prejudice from some who don't understand his condition. Along the way, he meets Harry Potter and his friends, who help him find a way to break the curse.
The plot could center around a magical chair. Harry and Remus find this chair in an old, abandoned castle. The chair has the power to call upon a unicorn and a wolf. When they sit on it, they are transported to a different dimension where the unicorn and the wolf are the rulers. Harry and Remus have to navigate this new world, understand the laws and culture there which are based on the relationship between the unicorn and the wolf, and find a way to get back to their own world.
Minerva McGonagall is also a favorite. Her Animagus form of a cat gives a lot of room for creativity. Fanfictions might show her observing Hogwarts in her cat form, seeing things that she wouldn't as a human, and using that knowledge to protect the school and its students.
In the context of the wizarding world, a unicorn animagus could have a connection to the prophecy. Maybe their existence is somehow related to the fight against Voldemort. They could bring a unique power or knowledge that helps Harry Potter and his allies. They might also be hunted by Voldemort's followers because of their special nature, which would create more drama and conflict in the story.
Obviously, Harry Potter would be one of the main characters. Since it's about him being an animagus.
A fanfiction titled 'The Animagus Legacy' might delve into the idea of an old Animagus family line that has been forgotten. A new student at Hogwarts discovers they are part of this line and must learn to control their Animagus form while also uncovering the secrets of their family's past and how it ties to the greater wizarding world.