THE HOUND HIMSELF
It's common knowledge that, in the novel, the hound was eventually killed.
Yet, the movie is vague about that. Holmes and Watson shoot at him as he's attacking Sir Henry, and scare him away. Holmes fires another shot, but nothing is indicated; as Holmes searches the moors with his torch, the audience never sees the hound's carcass, so maybe (on film) Holmes' shot missed.
Later, there was a reference that the beast was killed, but that could have been a ploy to entrap the real criminal--Jack Stapleton!
As an animal lover, I like to think that, in the movie, the hound escaped and lived out his life as a wild animal.
And, IMHO, if anyone deserved to have been shot to death, it's Stapleton! Pity the movie never showed that.