In 1978's Invasion of the Body Snatchers, alien beings are quietly taking over the human race by replicating each person in their sleep and replacing them with an imposter. Our human heroes have figured out that they can blend in by not showing any emotion. And it works!
... until the human-faced dog shows up.
Eff that thing to hell and back. Seriously.
Philip Kaufman's remake of the 1956 paranoid sci-fi classic of the same name was a fantastic update, tapping into a sense of post-Watergate distrust to create an exciting, boundary-pushing studio picture set against the backdrop of 1970s San Francisco. The film feels a lot like Francis Ford Coppola's The Conversation... but with plant-aliens.
That grounded reality only heightens the horror as the circle of actual human survivors shrinks and shrinks and shrinks until there's only a handful left.
Now, these aliens are in the habit of completely and accurately replicating people. The imitations are grown from pods and essentially suck the essence, the life force, from the humans as they sleep. These beings don't usually get it wrong, but when a homeless busker and his dog are being taken over as they sleep next to each other... well, all it takes is Donald Sutherland accidentally stepping on the pod growing their alien counterparts to create one of the most nightmarish images ever put up on a screen.
Up to this point in the movie we've seen some crazy stuff, including Donald Sutherland smashing his pod clone's face in with a garden hoe, but nothing as unsettling, as purely evil as the human-faced dog.
There are two reasons this creepy moment works so well. The first has to be the mask used for the dog's human face, which is the perfect mixture of real and surreal. It's a practical effect, one that was pretty on-par for horror films of the '70s, and you have to give credit to the special effects team for knocking it out of the park. It's clearly not a normal human face, but there's something to the design that makes it feel like it's more than just a mask, that's how uncanny it is. On top of that, Kaufman got lucky and the dog wearing the mask decided to shove its tongue out through the mouth hole, instantly making the effect an 11 on the "oh God, oh no" terror scale.
The other thing that makes the human-faced dog so creepy, aside from the mask itself, is the timing of the creature's appearance. When all hope seems lost our human group finally gets a leg up. They figure out that the only way these pod people can sense the humans from their own is through emotion, so if they can just walk among them calmly and without any emotion, they just might be able to make their escape.
And then that little bastard runs up and of course if you see that thing you have to scream. It's the only way to keep your mind from breaking right there on the spot, right? When they do react to that monstrosity they blow their cover and any chance of escape is out the window.
Not only is the human-faced dog the worst possible thing to see, but he also popped up at the worst possible time as well. It's a damn shame none of our good guys had a flame thrower on them because the only reasonable response to seeing a dog with a human head running at you out of the dark is to set it, its immediate surroundings and probably the whole city block on fire until there's nothing left.