Often, it could be a result of a specific magical event or encounter. For example, he might come across an ancient spell or a magical creature that somehow triggers the revelation of his animagus form. Maybe he's in a dangerous situation and his magic reacts in a way that reveals this new aspect of himself. This would add an element of excitement and unpredictability to the story. He could be facing a powerful dark wizard and in that moment of extreme stress, his animagus form emerges to save the day.
In some fanfictions, Harry's animagus form could be a stag, similar to his father's Patronus. It would be a cool connection to his family heritage and his own magical abilities.
His Animagus form could be a black dog. Black dogs are often associated with mystery and protection in the wizarding world, and Snape, with his complex and often secretive nature, could fit this form. He's always lurking in the shadows, much like a dog might when it's guarding or observing.
If Harry's animagus form is a large and powerful creature like a dragon, his familiar might be in awe of him. It could lead to a relationship where the familiar is very submissive and looks up to Harry for protection. For example, the familiar might be a little imp that hides behind Harry when they encounter other dangerous magical beings.
Another possibility is that he becomes a fox. Foxes are clever and sly, which are qualities that Harry has developed over the years. As a fox, he could use his wits to outsmart his opponents. He could also easily blend into the forest environment, which would be handy for hiding or sneaking around.
If Harry lives in his animagus form in fanfic, his identity would be a big mystery. People around him would only see his animagus shape. This could lead to some really interesting scenarios. He could use this to his advantage when sneaking around Hogwarts. Also, his friends might have a hard time recognizing him at first. But as they get used to it, they could come up with special signals or ways to communicate with him while he's in his animagus form.
In many fanfictions, the choice of animagus form often reflects the character's personality. If female Harry is hiding in her animagus form, it could be a form that represents her need for freedom and escape. For example, a bird. A bird can fly away from trouble and see things from a different perspective, which might be important for her in a difficult situation. She could be using this form to gather information without being noticed.
In this fanfiction, if Harry is Hermione's pet as an Animagus, he would likely use his animal instincts to protect her. For example, if there's a threat in the Forbidden Forest, his animal senses might pick it up before Hermione notices. He could also play a role in helping her with her research. Maybe he could lead her to some interesting magical creatures she's studying by leaving little hints in his animal form.
Friendship is also a common theme. Harry's friends' reactions to his Animagus form can range from shock to admiration. And sometimes, his Animagus form can even strengthen his bonds with his friends. For instance, if he can use his form to rescue a friend in a tricky situation, it deepens their friendship.