Maybe Dumbledore. He often has mysterious ways of providing Harry with tools or companions to help him in his journey. The trained hawk could be one such strategic gift from Dumbledore.
The plot could involve Harry using the trained hawk for various tasks like delivering messages secretly. Maybe the hawk helps him spy on the bad guys in Hogwarts, flying around unnoticed and reporting back to Harry.
The hawk could be initially shy around Harry's friends. But as time goes on, it might start to trust them. For example, Hermione might study the hawk's behavior and try to teach it some simple tricks. Ron, on the other hand, could be a bit scared of it at first but later grow to like it when he sees how useful it can be.
There could be many different authors who write 'harry is trained fanfiction'. Fanfiction is a very open - ended and community - driven form of writing. It could be written by an amateur writer who is a huge fan of the original work (presumably something related to a character named Harry). They might write it just for fun or to explore a different side of the character that they feel wasn't explored enough in the original.
It could be a deranged fan who has found a way to bring their dark fanfiction ideas to life. This person might be jealous of the original story and wants to create chaos in the wizarding world through their fictional concepts made real.
There's a chance it could be a powerful house - elf. House - elves are known for their own brand of magic, and one particularly rebellious or enlightened house - elf could decide to train Harry in magic that is more about stealth and quick - thinking. After all, house - elves have to be very sneaky to avoid detection by their masters most of the time, and this kind of skill could be very useful for Harry in his battles against Voldemort.
Maybe it's a former Death Eater who has seen the error of his ways. He wants to atone for his past sins by training Harry to defeat Voldemort. He has inside knowledge of the dark side's tactics and uses this to make Harry a more formidable opponent.
She could be an ancient Celtic goddess, like Brigid. Brigid is associated with magic, healing, and smithcraft in Celtic mythology. In the fanfiction, she might see Harry's potential and decide to train him in her unique magical arts.
Maybe the hawk has some special powers. For example, it can sense dark magic approaching. Harry then has to protect the hawk while also relying on it to keep himself and others safe. The hawk could also form a bond with Harry that allows them to communicate telepathically, which would add an interesting dynamic to the story.
In the context of the Harry Potter universe, the founders are Godric Gryffindor, Salazar Slytherin, Rowena Ravenclaw and Helga Hufflepuff. In this fanfiction, it's likely these are the founders who are training Harry.
It could be about Sirius teaching Harry advanced magic skills like Patronus charm. Sirius, being a skilled wizard, might train Harry to be better at dueling and facing dark wizards.