Another possibility is that it could be sent by an anonymous person who has somehow discovered his werewolf secret and is trying to intimidate him. This would add an element of mystery and danger to the fanfiction, as Remus would then have to figure out who sent it and why.
His mother is a likely sender. She could be worried about Remus and send a Howler to remind him to take his Wolfsbane Potion regularly or to be more cautious around others. Mothers are often protective, and in the wizarding world, a Howler is a rather forceful way to get your point across.
In the 'Remus gets a Howler' fanfiction, the Howler could be a misdirected one. It was actually meant for someone else but got sent to Remus by mistake. This could lead to all sorts of comical situations as Remus tries to figure out what to do with it. He might initially think he's in trouble for something he didn't do and start to worry. Then, when he discovers it's a mistake, he has to find a way to return it to the right person without getting into more trouble himself.
His mother could send the howler. In the wizarding world, mothers can be quite fierce when it comes to family reputation and she might be unhappy with Sirius's actions or friends.
Snape could also be a possibility. If Harry did something that Snape considered a serious offense in Potions class, like using an ingredient in the wrong way that could have caused an explosion, Snape might send a howler to scold him. However, it would probably also be filled with his usual sarcasm and dislike for Harry.
It could be Severus Snape. He has a complex relationship with Dumbledore. Maybe Snape discovers some of Dumbledore's plans that he doesn't agree with regarding Harry Potter. He then sends a Howler in a fit of anger to express his disapproval.
Perhaps another student, like Hermione, could send the Howler in a very creative twist in the fanfiction. Maybe she has found out that Draco is planning something malicious against the Gryffindors and uses the Howler as a way to threaten him into stopping.
Hermione could be the sender. If Snape has made some unfair judgment in class against Gryffindor students, Hermione, with her strong sense of justice, might send a howler to him to voice her protest.
It could be Professor Snape. He often had a contentious relationship with Harry, and if Harry did something that Snape thought was particularly wrong or against school rules in a way that affected Snape's interests, he might send a Howler.
Well, it could be that someone was really angry with him and sent it to express that rage.
Most likely, his parents, Harry and Ginny Potter, could send the howler. They would be concerned about their son's well - being and actions at Hogwarts. If Albus got into trouble or was not living up to the Potter family name, they would not hesitate to send a howler to set him straight.
His father, Harry Potter, might also send a Howler. Harry knows the importance of discipline and responsibility, and if James was shirking his duties or not following the rules at Hogwarts, Harry would not hesitate to use a Howler to communicate his disappointment and expectations.