Animagus
A Harry Potter story where one or more of the protagonists is an Animagus (and they weren't in canon), and where this is important to the story.
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In Care Of (Harry Potter) by Fangs Fawn
During the summer before sixth year, Harry finds an injured bat in the garden and decides to try to heal it...and an unwilling Snape learns just what kind of a person Harry Potter really is.