Crapsaccharine World: The epitome of this trope. Everything's all cute animals on the outside, hard-to-top horror on the inside.
And this was preferable to what they did at first. Tom says that they used to torture the new kids, and he's the one who came up with the idea to at least keep them distracted and happy until, and sometimes even after they cross.
Deconstruction: More or less showing just how disturbing finding oneself in a world full of Anthropomophic Animals and not being able to leave would be. To say nothing about how most of us would react to being confronted by such creatures in Real Life.
Fridge Logic: In-universe; Billy wonders how Penny could send a balloon message every day without being noticed, how she stole Nook's notebook, and how she escaped from Nook. He ignores all of this in favor of allowing himself to believe that he's finally found someone he can trust, but then you find out that Nook was the one behind the balloon messages all along...
Fridge Horror: An in-universe example. What do the gyroids turn animals into?
Genre Savvy: Billy, most of the time. For example, in the pro-box ending, while Penny monologues about the ways she could dismember Billy, he attacks her while she's still talking.
Hope Spot: The early balloon arc. Billy gets messages in balloons from a girl named Penny, someone who might be an ally; from this he starts piecing things together about the camp, and he eventually figures out that there are other kids and they might even be able to escape all together... then wham, Penny apparently gets captured and it turns out it was actually Nook sending the messages. Penny is the one running the whole freakshow, and Nook is the only friend he's got.
I Am Spartacus: Played with. Penny demands Billy to tell her who took her papers, or else someone from a crowd of kids gets it each time he lies. A boy in the crowd named Phillip claims it was himself. Penny takes him to the back of the house, and he gives her a gut wound with his sharpened crutch before she kills him.
Infant Immortality: Averted. Most of the animals started out as children just like Billy, and numerous other children are imprisoned and going through the transformation themselves. Billy notices a few children with missing limbs in the crowd near the end. And in the very same scene, Penny asks the crowd who it is that's trying to betray her. When no one answers, she takes a random little boy, rips him apart, and waves at the crowd with his severed arm.
Kick Them While They Are Down: After beating the crap out of Penny, she falls off the balcony, taking Billy with her. This is followed by Billy twisting his body around and executing a twelve-story body slam, then gets up and rips her eye out. The eye he rips out was his in the first place.
Multiple Endings: A rare non-video game example - Pro-Box or Anti-Box. Take your pick.
Pro-Box Ending: Billy opens Penny's final present, and finds out the truth about Penny, Tom, and the island. Not only that, but he manages to escape. The Golden Ending, essentially.
Anti-Box Ending: Billy runs away from Penny's final present, attempts to kill himself, tries to kill Tom, and crosses. If Pro-Box is the Golden Ending, then Anti-Box is the Bad Ending. Anti-Box is sort of a "what if" concering Billy getting lucky with his victory, Billy taking a while to cross, and doesn't really make sense unless you know that plot twist from Pro-Box.
No-Holds-Barred Beatdown: What Billy does to Penny at the end. Chewbot states that he chose this because of how brutally raw beating someone with your fists is.
Psychopathic Manchild: Billy, of a sort, in the anti-box ending. Notice how the tone becomes a lot more childlike after the Resetti segment.
Quote Mine: There's an instance of this to make Tom Nook seem more evil. Of course, most of the screenshots and dialogue is taken out of context (for example, in one of the endings, Mr. Resetti appears to chew Billy out for trying to drown himself when he's really yelling at him for resetting the game), and some of it is just Chewbot and other players sending messages that are clearly not what the game intended the system to be used for.
Billy: "Then... there's no Penny? She was never real?
Tom: "Oh, she's real alright. She's the one running this freakshow.
Also, in the form of the picture of that Billy pieces together and we realize he's on an island.
"Tom died unloved and alone in a dirty hole like an animal".
"Nook called it Animal Crossing."
"Where Are They Now?" Epilogue: The very last coda reveals that the story is actually an official transcript to the US government that Billy wrote. He is now almost thirty, has had intense psychiatric treatment, and has never returned home.
X Meets Y: Animal Crossing meets Silent Hill. Yahtzee probably did it first, but this one actually did it to even more disturbing levels.
Let's see... town with a dark secret, a strange bunch of animals who were once normal people, ominous artifacts that play with your mind and slowly mutate people, a mob that bursts into your house, beats you, rips one of your eyes out and then acts like nothing happened, a twisted psycho who is a borderline humanoid abomination... yep, that's what happens when Animal Crossing meets The Shadow Over Innsmouth... also H.P. Lovecraft in general.
Animal Crossing meets Lord of the Flies, in Tom Nook's own words.
Maybe some Island of Doctor Moreau, for good measure?
It's a Running Gag on this wiki and others that Animal Crossing is just like The Prisoner, but this version of the main character (Billy) shares Number Six's attitude about the second most important person not telling him enough, the most important person never showing up or speaking to him, and not being able to escape from town.