A slightly more up to date piece.

Nibiru was a pet from rural Wisconsin who lived near Lake Geneva. She had a tendency to wander off, one night going too far into the woods thinking there was maybe a hurt human after hearing shrill screams. The closer she got to the sound the less human it seemed, making her feel uneasy and disoriented.
Not knowing if she was starting to hallucinate or not she saw a pair of white eyes staring down at her. It seemed wrong because whatever they belonged to smelled like another dog, but were much higher up than any dog she had seen. The creature let out an unearthly scream, sound like a mix of a human in pain and a bear. It chased her deeper into the woods until she fell into a seemingly bottomless pit in a small clearing (similar to "Mel's Hole" ), dying in the process.
She later climbed back out not knowing what happened to her. Only remembering floating in a dark body of water far too big to have been the pit she was in. She tried to go back to her home, but when her family saw her they acted as if she was a monster. She wandered the fields near human populations for a while, occasionally trying to find another home only to get the same results. Eventually she went back to the woods feeling she had nowhere else to go.

Nibiru appears to be a normal dog but seems a bit "off" physically and emotionally. Her body is cold and her eyes have developed cataracts, but her vision isn't affected much. She also no longer produces blood, and can sustain significant damage to her flesh without feeling anything. Ex) Body parts can be removed and be reattached/heal via stitching; her limbs can grow back slowly if the original is not reattached, however she prefers not to due to the process being itchy and very irritating. She often has scars on her arms and legs simply because she hasn't realized she hurt herself.

She may be considered a "Dogman", a creature similar to a lycan with legends originating from the Great Lakes area of North America. She's slightly larger than normal for her breed and has a thicker neck and shoulders. At times she can take a bipedal stance in rather unsettling way for long periods of time and is able to maneuver her paws to do things such as open doors. Dying may have also made her a little unhinged at times, as she can go from being docile to snarling rather quickly.

She also occasionally vomits a black substance that smells like iron, it's unknown if that's what is left of her blood or something else.

TL;DR: May or may not be some kind of demonic/inter-dimensional zombie with freaky eyes.

Other than that, completely normal doggy!

Character was HEAVILY inspired by In The Woods Somewhere