When thinking about a fanfic like this, it's important to consider the different ways the author could blend these elements. For example, the chair could be a magical artifact left by some ancient wizard, and as an Animagus, Harry has a unique ability to sense or interact with it. The unicorn could be a link to a different dimension or a source of magic that Harry needs to tap into. This kind of fanfic could be a great exploration of the magical world beyond what we've seen in the original books, but finding it might require some patience and extensive searching through various fanfic repositories.
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.
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.
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.
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.
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.
Since it's a fanfic about Harry Potter as a Grim Animagus, it might start with the discovery of this unique ability. Perhaps there's an accident or some kind of special training that leads to him being able to transform into a Grim. In the story, the Grim form could have some special powers or characteristics that are different from a normal Animagus form. It could be that in this form, Harry has enhanced senses, like better hearing or smell, which helps him in his adventures. And it might also show how the wizarding world in general views this rather unusual Animagus form, whether they are afraid or in awe of it.
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.
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.
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.
Dumbledore could be his mentor. He has vast knowledge and experience, and he would likely see the potential in Harry being an Animagus. He could guide Harry through the difficult process of learning to control his transformation and use his Animagus form for good.