This list of mythical creatures that begin with this letter and I am constantly gathering information from each continent, and is continually being updated.
Gwyllgi (Welsh)
The gwyllgi compound noun of either gwyllt "wild" or gwyll "twilight" + ci "dog") is a mythical dog from Wales that appears as a frightful apparition of a mastiff or Black Wolf (similar to a Dire wolf) with baleful breath and blazing red eyes. It is the Welsh incarnation of the black dog figure of English folklore.
It appears as a huge mastiff, with a baleful breath and blazing red eyes which shine like fire in the night. One reputedly haunted the lane leading from Mousiad to Lisworney-Crossways, another a field called the Cot Moor, and a third the pit at Pant y Madog near Laugharne, Carmanthenshire. Like most giant killer ghost dogs, Gwyllgi takes the form of an enormous mastiff with flaming red eyes and deathly breath.