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.
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.
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.
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.
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.
Well, it's highly likely that there is. The Harry Potter fanfic community is vast and full of diverse ideas. Writers often explore unique combinations like this. In such a fanfic, perhaps Harry's Animagus form could have features similar to a basilisk, like the deadly stare or the ability to move through small spaces like a snake. He might also face challenges or advantages based on this form in the story, which could make for a very interesting read. You should start your search on popular fanfic sites and see if you can uncover this kind of gem.
One possible fanfic could be where Harry discovers he is an Animagus early on. His familiar could be a phoenix, for example. In this fanfic, the phoenix could guide Harry through difficult times, and their bond could be a source of strength against Voldemort. Harry could use his Animagus form, perhaps that of a stag like his father, to move around secretly and the phoenix familiar could provide additional protection and wisdom.
One great Animagus Harry fanfic is 'Harry's Hidden Form'. In this fic, Harry discovers he's an Animagus early on and it's a hawk. The story follows how he uses this new ability to his advantage in the wizarding world. It has some really cool action scenes where he scouts out Hogwarts as a hawk.
One interesting plotline could be Harry's discovery of his animagus form in a unique way. Maybe he doesn't follow the traditional path of learning in Hogwarts but instead has some sort of accidental transformation that leads him to explore his new ability.