This page will contain Spoilers for the ending of the game: if you plan on finishing the game then don't read past this point.
Endings as clearly stated are the games end, there are multiple endings instead of just one.
Escape with the dog
This contains spoilers for The Escape with dog Ending
At the end of the game, after choosing to go down a demonic tunnel instead of leaving the sewers and going through an area full of creatures, the player finds themselves in a hallway that progressively fades to white, the player goes back to a completely normal sewer exit, finding their dog outside of the sewers. The screen then fades again, showing the ending splash screen;
Just leave ending
This contains spoilers for The Just leave ending
At the start of the game, if you spam the E key while climbing on the ladder, you will get this ending. It's quite literally the same as the "Dead dog" ending, just without killing your dog.
Escape without the dog
This contains spoilers for The Escape without dog ending
At the end of the game, after choosing to abandon the dog and leave the sewers instead of going through a demonic tunnel, the player simply leaves the sewers, however without their dog, and the screen fades again to this splash screen;
Dead Dog ending
This contains spoilers for The dead dog ending
In the second, completely demonic zone of the Sewer Entrance just before entering lost tape#1 the player can go to a hallway to the right and grab a damaged handgun, then attempting to shoot the dog's shadow at the end of the hallway ends in the player being teleported to a sewer exit, however after leaving, the player's dog lies on the ground, saying negative things when interacted with, and a while later the screen begins to fade, then showing;
This ending is the hardest one to achieve,, since it requires certain steps to be taken.
This contains spoilers for The Nightmare Ending
First, the player needs to bring the corpse of Dr. Piccinini to Sotiris. Secondly, after killing The Siren, you must leave the area and go back later and retrieve a Child's Skull from the corpse of the Siren. Lastly, place the Child's Skull in the tree-looking entity named Bayagototh in one of the last areas. The player leaves the sewers after triggering the ending, however after leaving the player discovers that the city is filled with screams and sirens, the sun is completely blocked out, and everything being red.
There are some endings in the demos (I haven't gotten any of them but Kira has mentioned them)
This contains spoilers for the Pizza end... if you care about it
At the end of the sewers where you would get the handgun in the full game you will be prompted with three options if you pick teeth, hair, and eyes you will get the handgun. If you pick Pizza, Pizza, and Pizza you will get the pizza ending where it will transport you into a hallway that uses Spooky's Jumpscare Mansion textures where The Eyeless Hobo will chase you with a pizza and stab you with it, the goal in this ending is to run to the end of the hallway pick up the pizza and then run back without dying to the pizza wielding hobo. It will then end with a full image of the pizza poster from Spookys that says, "You Win!"