Maybe it was a jealous classmate who somehow managed to send a Howler anonymously. If Teddy was getting all the attention, like being really good at Quidditch or getting top grades, a jealous peer could have used dark magic or found a way to send a Howler to embarrass him in front of everyone at Hogwarts.
His mother's old friends might send a Howler. For example, if they thought Teddy was dishonoring his mother's memory in some way, like getting involved with dark magic or something that goes against the values his mother fought for. They would feel responsible to correct him and send a Howler as a very public and forceful way of doing so.
It could be one of his professors. If Teddy was constantly late to class or not doing his homework, a strict professor like Professor McGonagall might send a Howler to set him straight.
It could lead to all sorts of chaos and funny situations. Maybe it embarrasses him in front of his friends or reveals a secret he was trying to keep.
In the fanfiction, the Howler Teddy Lupin gets could be a case of mistaken identity. Some powerful magic could have misdirected the Howler meant for someone else to him. Teddy then has to figure out who the real recipient is while also dealing with the public humiliation of the Howler's scolding. He might start an investigation at Hogwarts, teaming up with his friends to solve the mystery and clear his name.
It could be Dumbledore. He was always one to oppose Voldemort and a howler from him would be a very interesting plot point. He might use it to taunt Voldemort or to try and make him second - guess his actions.
They are popular because Harry has a connection to Teddy through his father, Remus Lupin. Harry can act as a father figure to Teddy, and this relationship has a lot of potential for emotional depth. It's a way to explore the idea of family and mentorship in the wizarding world.
One possible scenario in this fanfiction could be that Teddy shows some unique magical abilities that remind Remus and Tonks of their own magical heritages. Maybe Teddy can transform in a way that combines both werewolf and metamorphmagus traits, which would be a really interesting plot point. It could also explore how Remus and Tonks, who had such a tragic end in the original story, get to experience some happy moments with their son Teddy through the magic of fanfiction.
Well, Teddy might be shocked by the Dursleys' coldness at first. He could show them some magic in a very subtle way, just to see their reactions.
Well, it could be about Teddy Lupin traveling back in time and meeting his father's friends, the Marauders. Maybe he gets to experience their pranks first - hand and learns some new magical tricks from them.
Perhaps the Marauders are summoned by some dark force in Teddy's time. Teddy, having heard of their adventures, helps them figure out what's going on. They teach Teddy some of their old - fashioned spells and tricks, while Teddy shows them new ways of magic. Together, they face the dark force and emerge victorious, with the Marauders having to find their way back to their own time.
Well, Teddy could learn a great deal about his family's history from the members of the Order. Maybe he'd discover some hidden secrets or untold stories.
One great fanfiction could be 'Teddy's Growth' which shows Teddy Lupin's journey through Hogwarts with Harry as a sort of mentor figure. It really delves into their relationship and how Teddy copes with being a werewolf, just like his father. Another one is 'Harry and Teddy's Adventures' where they go on various magical quests together, solving mysteries and facing dark wizards. It's full of action and their bond is beautifully portrayed.