The translation means "devil dog" or "dog of darkness". This is a creature that stalks the Welsh country side at night. It is a type of Hellhound.
In the Welsh stories, the Gwyllgi is the devil himself, chasing his victims. Those who escaped must be Olympic runners, because who else could out run the devil?
It is said to have a heavy coat, though the descriptions are varying. They glowing red eyes remain the same. It is almost like the fires of hell itself come out of this creature's eyes.
These beasts are well documented around Wales and many an isolated road is named after this demon dog. Lon Bwbach Dbu ("Lane of the Black Spector") is located out side the village of Marchwiel, North Wales.