The Three-Eyed Cat is an NPC that can occasionally be found in Save Rooms in New Game+. It's modeled after Kira's (the developer's) user picture, so it stands to reason it represents the developer watching the player's progress.
Hitting the cat with a weapon will transport you into a purple void with feline eyes opening all around, triggering a game over shortly after.
Three Eyed Cat
Three Eyed Cat's death screen
Three Eyed Cat sleeping
Maggot Coffin[]
The Maggot Coffin is a hidden coffin filled with a fleshy mass with maggots wriggling over it. It's hidden in Siren's Lair, directly opposite the hall from the heavy door that is used to crush the Siren and defeat it, there's a rusty shutter door. Waiting in front of it for five minutes will unlock it.
When interacted with, the Maggot Coffin will speak:
A girl was lost from self-image and doubt. A boy was lost with paranoia in his hide out. Another soul watched them, lost as well. They followed down to the 7th hell. In the second ring they fought their own fear. In the guise of finding their pet most dear.
Maggot Coffin
Opened Maggot Coffin
Pizza Hands[]
There's a small secret area near the start of the game, in the corridor where your dog is getting pulled away from you. A segment of the left wall can be passed through, revealing a small room with hands protruding from the floor, holding pizza slices.
Pizza hands
Minor Secrets and Easter Eggs[]
The underlined letters in the Nezumi Testing sign spell out "Nesting".
Sotiris can be very briefly seen in The Mines: first at the start and second after climbing a ladder to escape the Dog Mimic.
Demo Crispys, the enemies from the original demo, can appear in randomized enemy mode.
A picture of Specimen 8 (Deer Lord) from Spooky's Jumpscare Mansion can be found in the Subway and in Nezumi Testing.
Mymadnessworks
One of the graffiti in The Sewers depicts White Face from IMSCARED, a game similar to Lost in Vivo in that it is also a first person horror with faux retro aesthetics and meta elements, created mostly by one person.
In Lost Tape 3 (The Cabin), right before burning the idol, Specimen 8 from Spooky’s Jumpscare Mansion can be seen outside the window, peering into the building.
Specimen 8 looking through the window
I have the body of a pig[]
In Lost Tape 3 (The Cabin), a graffiti on the wall says "I have the body of a pig". That's a reference to a ghost video where this phrase can be heard uttered in an otherwise empty basement.
Bayagototh shows up in Midnight Mode when the player has 30 coins and rings the bell. He comes with a new model and all. His midnight model can be found in the Gallery behind the other Bayagototh model.
Demi[]
Demi is a character in the first person dungeon crawler, Lunacid by Kira, but she originally makes an appearance in Midnight as a painting.
In Lost Tape 5 (The Wolf), if the player looks directly into the moon for long enough, they will be transported into a series of pulsating corridors. A radio can be found there, and looking into the radio for a while will make it play a strange broadcast:
Can you hear it?
[another voice] Hear what? That sound. It's like... It's something I can't even describe! It's... It's singing! And it's singing only to me. Just to me. Can't you hear it?
The end text after the moon sequence is over
Looking at the earth through the moon
The Wolf Attacks[]
In Lost Tape 5 (The Wolf), waiting around after lighting up the fire will cause the Wolf to appear early and end the tape right then and there.
The Wolf Talks[]
In Lost Tape 5 (The Wolf), the Wolf can actually talk to the player. To enable this, the player has to find an altar in the woods and interact with it until Bayagototh's scream can be heard. The Wolf will appear during the driving section as normal, but this time it will speak:
There are two of us but only one shall be fed One will consume The other shall dissolve
Looking at the altar in the woods
The Wolf talking to the player
The Backrooms[]
In Lost Tape 6 (The Way Out), if the player walks into the wall opposite their cell door and right of the stairs, they can phase through the wall to enter a yellow building that loops forever.
In the back rooms the player will wander a small looping maze-like map with some Source Engine jokes, buttons, and weird ambiance. Awhile after first entering the area, an unknown player by the name of Rancid Johnny will join the server, will then be made admin, and will proceed to start killing the player's steam friends along with sending the player threatening messages in 'chat'. The player's steam friends will also call for help sometimes. This is all a ruse though, as there is not goal within the backrooms, nor will Rancid Johnny or any other player ever actually appear.
A button in The Backrooms
The server rules and Rancid Johnny joining
Rancid Johnny being made admin
A missing texture and 3 steam friends dying
Rancid Johnny talking on voice chat
Trivia[]
The Pizza hands are a reference to the pizza ending in the demo.
Specimen 8 is implied to have been created by Bayagototh.
Prior to an update, the Three Eyed Cat could show up outside of New Game+.
Jarren Crist (the composer) regrets not being commissioned to change all of the instruments in the save room theme to meowing when the cat is present.Source
The Wolf attacking you if you wait too long after starting the fire is due to one of testers deciding to hide in the tent and wait out the storm.
