Harry Potter himself could be the one. In a situation where Draco has been acting especially maliciously towards Harry and his friends, Harry might find a magical way to lead Draco around on a leash as a sort of payback. But as he does this, he realizes that Draco is more complex than he thought and starts to feel a bit guilty about the humiliation he's causing Draco. This could lead to a whole new arc in their relationship where they start to see each other in a different light.
A new character could be introduced, perhaps an exchange student from a different wizarding school. This character has different views on discipline and magic and sees Draco as a prime example of a spoiled wizard. So, they use their unique magic to lead Draco around with a leash as a way to teach him a lesson about humility and respect for others.
It could be Hermione. Given her strong - willed nature and her often - clashing relationship with Draco, she might be the one to put him in such a situation. Maybe she discovers a new spell or magic item that allows her to do this as a way to keep Draco in line during a particularly tense time at Hogwarts, like when there are threats from dark forces and she doesn't trust Draco not to side with the wrong people.
One possible plot could be that in an alternate universe where wizarding traditions are very different, Draco has committed some minor offense in Hogwarts. As a form of punishment, he is made to be led around by a leash by the protagonist, who could be a Gryffindor student. This would lead to a lot of power - play and interesting character dynamics as Draco, who is used to being in a position of privilege, has to endure this humiliation. Along the way, they might start to understand each other better and form an unlikely friendship.
One idea could be a story where Draco has lost his memory and the character leading him around on a leash is actually trying to protect him from the dark forces that are after him. They use the leash as a way to keep him close in this dangerous situation.
Harry Potter could be one of the main characters. He might be the one who discovers Draco in this strange situation and has to decide whether to help him or leave him be, given their past rivalry.
The plot could involve a character who discovers Draco, a mysterious and powerful creature, and decides to keep it on a leash for safety reasons. But Draco doesn't like being restrained and tries to break free. This leads to a series of adventures as the character tries to keep Draco under control while also uncovering the secrets of Draco's origin.
One possible plot could be that Draco is in a situation where he is being controlled by a leash for some magical or political reason. Maybe he's being paraded as a symbol of a new order in the wizarding world. His former Slytherin pride is put to the test as he has to endure this public display.
He might become more submissive at first. Having to be led around on a leash is a very humbling experience for someone as proud as Draco. But as the story progresses, he could develop a sense of rebellion against this forced situation.
One possible fanfiction could be about a magical creature Draco that is tamed and put on a leash by a powerful wizard. The wizard might be using Draco for some sort of mission, like exploring a dangerous enchanted forest. Draco could be a dragon - like creature with its own will, and throughout the story, it struggles against the leash both physically and emotionally as it tries to assert its independence.
Draco and a harness leash could be part of a story where he's in a magical circus. He's in charge of some magical animals and the harness leash is what keeps them in line. But Draco might start to have second thoughts about keeping these animals in captivity, and the harness leash becomes a symbol of his internal struggle between duty and compassion.
One possible fanfiction could be about Draco using a harness leash in a magical creature training scenario. Maybe he's trying to tame a rare and unruly beast, and the harness leash is a special magical tool. He has to use his Slytherin cunning to figure out how to work with the creature while using the leash effectively.
In 'leash on draco fanfiction', it could center around a power dynamic. Draco is often seen as a proud and somewhat haughty character. So having him on a leash could represent a shift in power. For example, another character might be in charge of him, and this could lead to all sorts of interesting interactions, like the character using the leash as a means to make Draco do things he wouldn't normally do, or it could be a part of a redemption arc where Draco has to submit to certain conditions to grow as a person in the fanfiction world.