Apocalyptic horror from an animals POV was always something I wanted to see but is surprisingly is rare.
Yes that monster was something I made. While most of the sprites in that prototype were placeholder assets I either purchased or came with rpg maker, (although I did modify many of them), most of the monsters (and the humans, deer, owl and snake) were sprites I made.
The monster your thinking of (the "hideous beast" that came up behind the girl, and later startled the fox) was a photo-manipulated image. The base image was uncooked mutton being sold at a market deli (certain dishes, such as Kash and Smalahove, use sheep heads) I found on flickr under a creative commons license.
I don't have an explanation for what the creature is supposed to be or where it (or any of the monsters) came from. This was intended to make the setting more mysterious and foreboding, as well as emphasize that the animals have no way to understand what's happening.