The plot could start with Harry stumbling upon Cerberus in a hidden part of Hogwarts. He feels an instant connection and decides to try and make Cerberus his familiar. This would be a difficult task as Cerberus is known to be very fierce. But Harry's kindness and his ability to understand magical creatures might win Cerberus over.
One plot idea is that in the Triwizard Tournament, instead of facing the real Cerberus guarding the Philosopher's Stone, Harry befriends it. After that, Cerberus becomes his familiar. This changes the course of the tournament and also his relationships with other students. Some might be jealous of his new powerful familiar, while others might be in awe. And it could also lead to new challenges as Voldemort tries to use Cerberus against Harry.
Perhaps the fanfic could explore the idea of Cerberus being a familiar that helps Harry in his fight against Voldemort. Cerberus could guard important places for Harry, or even accompany him into battle. Its three heads could provide different forms of defense or attack, making it a very formidable ally for Harry in the wizarding war.
One common theme is the idea of an outsider or different type of magic. Since a Cerberus as a familiar is not a common concept in the original Harry Potter series, it often represents Harry or another character being different. For example, they might be shunned by some for having such an unusual familiar.
There's also a fanfic called 'Cerberus by His Side'. In this story, the Cerberus becomes Harry's protector in a more dangerous version of the wizarding world. It's really interesting to see how the author weaves in the lore of Cerberus from Greek mythology into the Harry Potter universe. The Cerberus is depicted as a fierce yet loyal companion to Harry.
Perhaps in this fanfic, there is some sort of danger in the wizarding world that can only be overcome by Harry and Remus having this special familiar - master relationship. Harry might turn Remus into his familiar to protect him from a dark force that is specifically targeting werewolves. As they navigate this new dynamic, they also have to deal with the reactions of their friends and the wizarding community at large. The story could explore themes of acceptance and the power of unconventional relationships.
Well, one possibility is that Harry discovers a new magical ability that allows him to make a familiar. He chooses Remus because of their strong bond. Then, they might have a deeper connection in battles, with Remus being able to sense Harry's needs and intentions more acutely. Maybe Remus could also gain some new powers or abilities as Harry's familiar, which would be really interesting to explore in the fanfic.
In a possible plot, the magical bond of being a familiar could give Harry and Snape a new level of understanding of each other. Harry, as Snape's familiar, could gain insights into Snape's past and his true motives. Snape, in turn, would have to come to terms with having Harry so close in this new capacity. This could lead to a series of adventures where they work together in a different way than in the original series. For example, they might have to face Voldemort together with Harry using his new familiar - related powers and Snape using his potions and dark arts knowledge.
Well, perhaps the plot could involve Harry finding a dragon egg and raising the dragon as his familiar. This would bring a whole new set of adventures as he has to hide the dragon from the Hogwarts faculty who might not be too keen on a dragon on campus. He could also use the dragon's special abilities to his advantage in his battles against Voldemort.
One possible plot could be that the girl is lost in a mythical realm where Cerberus guards. At first, she is scared of Cerberus, but gradually, through some kind of magic or special connection, Cerberus becomes friendly towards her and gives her a lick as a sign of acceptance.
The plot could involve the wolf familiar helping Harry in his battles against Voldemort. Maybe the wolf has special senses that can detect dark magic, allowing Harry to be one step ahead. For example, it could sniff out Death Eaters' hiding places.
One possible plot could be a young wizard accidentally summons a sucubus as a familiar. The sucubus, instead of being purely malevolent, has her own motives and secrets. She could help the wizard in his adventures at Hogwarts while also causing some chaos due to her nature. For example, she might use her charm to get out of tricky situations or to get information from other students or teachers, but this also gets the wizard into some trouble with the school authorities.