The Black Dog legend, usually found and heard from around the Britain is often associated with hellhounds. As i have said in a post before about black dogs, they are often harbingers of something bad. Once you've seen one, it is unmistakable. They are huge and are comparatively larger than the average dog. Almost always their ears are alert and their eyes glowing red. Often times these creatures are spotted in crossroads, which is also relevant to myths regarding the devil, and also ancient execution and/or burial grounds.
Once thought to be exclusive to the British Isles...black dog sightings are now widespread across the world. And unsurprisingly so, almost all cultures regard it as evil. Common sightings outside of Britain include: southern regions in the US, latin america, south east asia (especially the philippines), and China.
No one really knows what they want or what their purpose here on our plane of existence, but some say they are signs of the devil and the end times.
Sherwood, Simon J. (2010) Apparitons of Black Dogs in Smith, Matthew D. (ed.) Anomalous Experiences: Essays from Parapsychological and Psychological Perspectives, McFarland, ISBN 978-0-7864-4398-7