We learn from Harry Potter and the Chamber of Secrets that the young Tom Riddle convinced Headmaster Armando Dippet that Hagrid was Slytherin's heir, and that he was responsible for the death of Moaning Myrtle. Did he identify any particular monster as the device for killing? Did he blame Aragog the Acromantula (giant spider), or the werewolf cubs? The Trolls in the Forbidden Forest perhaps? If so, how would Slytherin and his Chamber of Secrets have been involved?
It seems likely that they believed it was Aragog.
When Harry and Ron go to see Aragog in the Forbidden Forest, he tells them that he was suspected to be the creature:
“But that was years ago,” said Aragog fretfully. “Years and years ago. I remember it well. That’s why they made him leave the school. They believed that I was the monster that dwells in what they call the Chamber of Secrets. They thought that Hagrid had opened the Chamber and set me free.”
— Chamber of Secrets, chapter 15 (Aragog)
This seems very plausible, and would be an easy lie for Riddle to spin. It would be harder to convince people that it was werewolf cubs:
- JK Rowling has said that Hagrid didn’t have them, so it would be difficult to provide any evidence of their involvement.
- The attacks aren’t characteristic of werewolves – none of the victims would have shown signs of lycanthropy, they were turned to stone. And they would have seen Myrtle’s body, which wouldn’t have any scars from a werewolf attack.
- As a species, werewolves seem to be better understood than Acromantula. It would be easier to paint an unusual spider as behind mysterious attacks than a werewolf cub.
It’s unclear whether Hagrid was believed to be the Heir of Slytherin.
When Harry meets the memory of Riddle in the Chamber, he seems to acknowledge that the myth of an Heir was floating around when he was at school:
“I admit, even I was surprised how well the plan worked. I thought someone must realize that Hagrid couldn’t possibly be the Heir of Slytherin.”
— Chamber of Secrets, chapter 17 (The Heir of Slytherin)
Apparently nobody expressed doubt that Hagrid was the Heir, but that doesn’t mean they thought he was, either. The story is almost a thousand years old; it would be a surprise if the modern version was wholly accurate. Perhaps people thought the part about a chamber full of monsters was true (and that’s what Hagrid found), but the part about the Heir was made up. We don’t have enough information to say, I think.