A possible plotline could be that Harry is turned into an animal form by Voldemort's dark magic and is kept as a pet. As a pet, he overhears Voldemort's plans and uses this knowledge to his advantage. He might also form an unexpected bond with some of Voldemort's followers who start to question their master's actions because of the way he treats Harry. This could lead to a complex web of alliances and betrayals within the Death Eater ranks.
Perhaps Voldemort uses magic to brainwash Harry into thinking he is his pet. Harry then has to fight against this false identity while still being under Voldemort's control. The plot could involve Harry slowly regaining his true self and using his new - found knowledge of Voldemort's inner workings (from being so close to him) to defeat him in the end. It would be a journey of self - discovery and resistance.
One possible plotline could be that Voldemort's pet somehow steals Harry's glasses as a way to taunt him. This could lead to Harry going on a quest to retrieve them, facing various dark magic obstacles along the way.
One possible plotline could be Harry being held captive by Voldemort in a way that's like a pet. Voldemort might use Harry to show his power over the Boy Who Lived. Another could be that Harry has some sort of magic that makes Voldemort keep him as a pet instead of killing him immediately.
One possible plotline could be that Voldemort captures Harry early on and instead of killing him, decides to keep him as a pet. Harry, being in this strange situation, might start to develop a warped sense of loyalty to Voldemort over time.
One plotline could be that Voldemort wants to use the marriage to gain control over Harry's special powers. Another could be that it's part of a prophecy that neither of them can avoid. Or maybe it's a political move in the wizarding world to end the war between the two sides.
One possible plotline could be Harry trying to find a way to break the forced marriage bond while secretly gathering allies to fight Voldemort.
Perhaps Snape, in a moment of weakness or out of some deep - seated sense of responsibility, decides to protect Harry in an extreme way. He treats Harry like a pet in that he provides for all of Harry's needs but also restricts his freedom. This could lead to a lot of internal conflict for Harry, who is used to being independent. Meanwhile, Snape has to deal with the other characters in the wizarding world who might not understand his motives for treating Harry this way.
One possible plotline could be that Harry is tricked by Voldemort. Voldemort might use some powerful dark magic or false promises to make Harry believe that joining him is the right thing to do. For example, Voldemort could claim to have a way to end all the bloodshed in the wizarding world if Harry helps him.
One possible plotline could be a magical accident that causes Harry to be magically bound to Voldemort in a way that he has to 'ride' him, like being attached to his back or something. This could lead to a situation where Harry has to endure Voldemort's actions while trying to find a way to break free. And during this time, he might gain some insights into Voldemort's thoughts and plans, which he could later use against him.
One possible plotline could be Harry initially being very distrustful of Voldemort, but as time goes on, he starts to see a different side of him. Voldemort, on the other hand, might be trying to use Harry for his own purposes at first but then develops a real bond. Another could be that Voldemort has to face the wizarding world's reaction to his newfound role as Harry's protector, which could lead to conflict. And Harry could grow to become a powerful ally to Voldemort in a new way, changing the power dynamics in the wizarding world.
One possible plotline could be that Harry has to hide this fact from his friends at Hogwarts, fearing their reaction. He might struggle with feelings of guilt and confusion, especially if he starts to show some of Voldemort's darker tendencies.