Perhaps Draco could be a Kneazle. Kneazles are intelligent and independent, which would suit Draco's personality. As a Kneazle, he might be constantly sneaking around and getting into places he's not supposed to. Harry would have to use his wits to keep Draco out of trouble and also earn his trust, as Kneazles can be quite picky about who they bond with.
There's 'A Different Kind of Love'. In this fanfiction, the idea of Harry being Draco's pet is a starting point for a very slow - burn romance. It shows how they first misunderstand each other but then start to see past their differences as their feelings grow. The author does a great job of building the tension between them.
One possible plotline could be that Harry, in a strange magical turn of events, is transformed into a small animal and Draco finds him. Draco then decides to keep him as a pet. Another could be that it's all part of a secret spell experiment where Draco has to take care of Harry in pet form to learn about empathy.
There's also 'The Pet Project with Draco'. In this story, Draco gets involved in a school project related to pets, and it's interesting to see how his character develops throughout the challenges he faces in the project. It has a great mix of drama and humor.
A common theme is power and control. Harry being in the position of master over Draco brings up questions about who really has power in their relationship. Is it Harry because of his new role, or does Draco still hold some power through his Slytherin cunning? Another theme is redemption. Draco, in the role of a pet, has a chance to redeem himself from his past actions as a bully.
If Harry is Salazar, he might have enhanced Parseltongue abilities. Salazar was known for his connection with snakes, so Harry could communicate with a wider variety of serpents and perhaps even gain more control over them. For Draco as Godric, he could have a stronger affinity for Gryffindor - like magic, such as a more powerful Patronus, perhaps in the form of a lion, representing Godric's courage.
Harry, even as a pet, would likely still have his brave and curious nature. He might be constantly trying to figure out a way to break the spell or understand the situation better. Draco would probably be his usual haughty self at first, but gradually show a more caring side as he gets used to having Harry as a pet.
It could be a hippogriff. Hippogriffs are very majestic and intelligent magical creatures. In the Harry Potter series, Buckbeak was a hippogriff that Harry had an encounter with. It has the body of a horse and the wings and head of an eagle, which makes it both powerful and graceful.
One magical element could be an enchantment that allows the fan to adjust its speed based on Harry's body temperature. So if Harry is really hot, the fan speeds up automatically. Another could be a charm that makes the fan blend in with the Hogwarts decor when not in use, like it turns into a small quill or something. Also, Draco might use a spell to make the fan levitate slightly, so it can follow Harry around and keep him cool wherever he goes in the room.
There's a lesser - known piece called 'The Taming of Draco' which is really interesting. It shows how Harry, in this rather unconventional role of master, has to deal with Draco's pride and how Draco gradually comes to accept his new position. It's a story full of humor and some really touching moments.
Another reason could be for the pure fun of it. Fanfiction writers like to play with different ideas and tropes. This particular trope gives them a chance to create a unique universe where the characters can fall in love in a way that is completely different from the original story. It also allows for a lot of emotional exploration as the characters adapt to this new relationship dynamic.