Cŵn Annwn

In Pagan folklore, the Cŵn Annwn are the ghostly hunting hounds of King Gwynn ap Nudd, the dark king of the faerie world. They are large, white hounds with blood-covered red ears.
The Cŵn Annwn can only be seen or heard by the dead and those about to die. To hear the barking and mournful howling of the hounds means that the faerie king is coming to take one's soul to the Otherworld. Once the barking is heard by a living person, the listener's death is imminent and their fate is inescapable.
Sir Gwyn the Witch Hunter saw the Cŵn Annwn before death.