Why is "The Canterville Ghost" a mix of comedy and horror?
"The Canterville Ghost" is a horror story because parts of the story are genuinely creepy and scary. In the story, readers are told that Sir Simon brutally murdered his wife and has been haunting the house for generations. Additionally we learn about a blood stain that can't be removed. It reappears despite all best efforts to clean it. Further scares come from the ghost himself. When he first appears to Mr. Otis, Sir Simon is dragging chains with him and has burning red eyes.
The story is a comedy because despite Sir Simon's best efforts, the Otis family flat out refuses to be afraid. It's not just that they put on a brave face either. They are literally not scared in the slightest. For example, Mr. Otis is more annoyed with Sir Simon that first night than scared. That's why Mr. Otis hands the ghost some oil and asks him to kindly keep the noise down. The twins end up antagonizing the ghost. They set up trip wires for him, they throw pillows at him, and they even scare the ghost with fake ghosts. Sir Simon spends most of the story plotting new ways to scare the Otis family only to be defeated in a funny way.