The Far Side always seemed to make jokes about animals thinking about things the way humans think about the animals. Example, a cockroach having a nightmare about a shoe descending towards it. But cockroaches probably don't even know what shoes are -- only humans know about smashing cockroaches with our feet.
Horror movies are sort of the opposite. Rather than having everything think about things the way humans do, everything is always doing whatever it does, but it always affects humans.
So, a guy rises from the grave to get revenge on his children. But why is it a human that rises from the grave? Could a chipmunk rise from the grave? If that happened, it would be a comedy.
Or an animal is in a crate, and it eats people. But, uh, animals CAN eat people. There's nothing supernatural about it. Apparently, what makes it a horror movie is that, in this movie, it specifically eats people. If it ate a penguin, it'd be a nature documentary.
