Wikipedia
The cadejo is a character from Central American and southern Mexican folklore. There is a good white cadejo and an evil black cadejo. Both are spirits that appear at night to travelers: the white to protect them from harm during their journey, the black (sometimes an incarnation of the devil) to kill them. The colors of the cadejo are sometimes exchanged according to local tradition. In some places, the black cadejo is seen as the good one and the white cadejo the evil one. They usually appear in the form of a large (up to the size of a cow), shaggy dog with burning red eyes and a goat's hooves, although in some areas they have more bull-like characteristics. According to the stories, many have tried to kill the black cadejo but have failed and perished. It is also said that if a cadejo is killed, it will smell terrible for several days, and then its body will disappear. Some Guatemalan folklore also tells of a cadejo that guards drunks against anyone who tries to rob or hurt them. When the cadejo is near, it is said to bring about a strong goat-like smell. Most people say never to turn your back to the creature because otherwise you will go crazy. Speaking to the cadejo will also induce insanity.
In popular etymology, the name cadejo is thought to have derived from the Spanish word "cadena", meaning " chain"; the cadejo is at times represented as dragging a chain behind him. There is a fairly large member of the weasel family, the tayra, which in common speech is called a cadejo and is cited as a possible source of the legend.
In El Salvador, the dog-like creature is known as El Cadejo, is said to look like a dog but is not a dog, has deer-like hooves and also moves like a deer, rather than a dog. The white Cadejos are known to be benevolent and eat bell-like flowers that only grows on volcanoes. The white Cadejo protects people including drunks, vagabonds, and people with grudges from all evil foot steps and even La Siguanaba, and bad choices which are sometimes caused by the evil black Cadejo. The black Cadejo is malevolent and lures people to make bad choices. The black Cadejo has glowing red eyes and eats new born puppies and sometimes the black Cadejo is said to be the devil himself. The black Cadejo is said to be able to stand on two feet like a man and swiftly throw punches on its victims, letting them know that they are no ordinary dog. The novel "Los perros magicos de los volcanes" (Magic Dogs of the Volcanoes) by Manilo Argueta, describes the Cadejos as mythical dog-like creatures that figure prominently in the folklore of El Salvador. They mysteriously appear at night and lovingly protect the villagers who live on the slopes of the volcanoes from danger. In El Salvador the legend of El Cadejo revolves around La Siguanaba and El Cipitio legends.