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.
The Howler might contain Harry's anger and defiance. He could be yelling about all the harm Voldemort has caused, like the deaths of his parents and many others. It could also include threats of how Harry will stop him once and for all.
One possible fanfiction idea could be that Molly's howler in Harry Potter was actually a warning not just to Ron, but also a veiled threat to a dark wizard who was secretly infiltrating Hogwarts. Maybe this dark wizard had a plan to use Ron as a pawn, and Molly sensed it through her motherly instincts and magical awareness. So the howler was her way of sending a message to both Ron and the hidden enemy.
In some Harry Potter fanfiction, Ron's muscles might be emphasized in various ways. For example, in a story where the trio has to face a physical challenge, Ron could suddenly show off his unexpected strength due to his well - developed muscles. Maybe he has been secretly training during their time at Hogwarts, and this newfound strength helps them get out of a tight situation.
It could be that someone did something really wrong and Harry needed to express his strong feelings in a very public and intense way. Howlers are notorious for being attention-grabbing and forceful.
Voldemort might initially be furious. He's not used to being challenged in such a blatant way. He could try to destroy the Howler immediately in a fit of rage.
Perhaps Harry was feeling betrayed or misunderstood and wanted to express his strong emotions directly to Dumbledore through a howler. This could be a way to make his point and demand a response.
It could be that Molly crossed a line with Harry, and he felt that a howler was the only way to get his point across and make her understand his perspective. Maybe it was a big misunderstanding or a serious rule violation on Molly's part that triggered Harry's reaction.