Well, it could be about a character who is forced to hide their true identity and pretends to be a pet in the wizarding world. Maybe they are on the run from some dark wizards and find a safe haven by disguising as a pet in a Hogwarts dormitory.
It could be a young wizard who is an outcast in the wizarding world. Maybe they have some unique powers that others fear, so they choose to hide as a pet to avoid persecution.
One possible plot could be that a new, mysterious pet appears in the pet shop in Diagon Alley. This pet has some unique magical abilities that no one has ever seen before. A young witch or wizard discovers it and tries to figure out its origin and how to take care of it. Maybe it's a key to an ancient magic mystery.
It could also be a more complex plot where Harry is under the influence of a dark spell that makes him commit the act of killing, and he has to hide the body while struggling to break the spell. He may be in a race against time, not only to hide his deed but also to find a way to reverse the spell before it's too late.
A possible plot could be that a powerful dark wizard has enchanted a Nundu to attack Hogwarts. However, one student who has a special connection with animals manages to break the spell on the Nundu and turns it into a pet. This Nundu then becomes a protector of Hogwarts, using its strength to fend off other dark creatures. Along the way, the student has to deal with the prejudice of others who fear the Nundu due to its reputation.
In a Harry Potter Nundu pet fanfiction, the Nundu might be a gift from a mysterious figure. The protagonist, who has always been an outcast among the wizarding community, forms an unbreakable bond with the Nundu. They embark on a journey to find a way to make the Nundu's venom, which is deadly, into a powerful potion ingredient that can save the wizarding world from a new dark threat. Along the way, they face many obstacles and make new friends who initially fear the Nundu but later come to respect it.
Maybe in this fanfiction, Harry Potter is turned into a pet dog himself by a misfired spell. As a dog, he experiences Hogwarts in a whole new way, making friends with other animals and using his dog - like senses to uncover secrets in the castle. He might even have to find a way to turn back into a human while still dealing with all the normal Hogwarts drama.
Well, the plot could involve the fox having some special powers that help Harry in his adventures. For example, the fox might be able to sense dark magic from afar. Harry would then have to keep the fox's existence a secret from some of the teachers who might not approve of such an unusual pet.
One possible plot could be that the dragon is a gift from a mysterious wizard. Harry at first is hesitant but soon grows attached to it. The dragon has special powers that draw the attention of both the good and bad in the wizarding world. Harry has to train the dragon and at the same time, deal with the political implications of having such a powerful creature. His friends are both in awe and a bit scared of the dragon, which also creates some tension in their relationships. And as the story progresses, the dragon becomes a key factor in a major battle against dark forces.
In fanfiction, Harry Potter could have a phoenix as a pet. A phoenix is a very magical and majestic creature. It can be a great companion for Harry. It has the ability to heal itself by bursting into flames and being reborn from the ashes, which could be useful in many dangerous situations Harry might face.
Perhaps in this fanfiction, Harry is actually a pet in a magical, alternate reality where Severus is a powerful wizard with a penchant for collecting unique magical creatures. Harry, in his pet form, might hold some sort of special power or significance that Severus is eager to uncover. This could lead to adventures as Severus tries to unlock the secrets within Harry while also keeping him safe from other dark wizards who might want to steal Harry for their own purposes.