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. please.
Endings as clearly stated are the games end, there are multiple endings instead of just one
Escape with the dog
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;
Escape without the dog
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
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 fading, then showing;
This ending is the hardest one to achieve, since it requires certain steps to be taken.
First, the player needs to bring the corpse of Dr. Piccinini to Sotiris. Secondly, killing The Siren, you must leave the area and go back later. Lastly, using the Child item in the tree entity in one of the last areas. In this ending, Bayagototh will be summoned and its horrors will spread across the world, Suffocating the world. The player leaves the sewers after triggering the ending, however after leaving the player discovers that the city has police and ambulance sirens going about, the sun being completely blocked out, and everything being red.