The Fire Dog is a dog that was caught on fire while the Hunter Quarry was still in operation, most likely in an accident involving an explosive. The Dog survived the incident and can typically be seen still roaming around the Quarry at 21:00. The Fire Dog is peaceful and will only run away and tries to stay out of the player's field of view. The myth was originated by a YouTube user named TheAlbert17. He claimed that a friend took a picture of a dog in the Quarry. He did in fact find something, but most of the myth hunting community believed it to be an edited video, so there has yet to be any more substantial evidence. The myth is considered dead and false, as the creator was debunked as being a hoaxer some time later. This picture was proved as a Photoshop as well.
A suitable possibility, which makes this myth a dis-belief, is, that the object in fire, believed as the fire dog, might be in fact a part of an airplane due to the frequent plane crashes that happen in or around Hunter Quarry.
So, the object believed to be the Fire Dog might be just the burning remain(s) of the crashed plane that might have collided with something.