In Welsh mythology and folklore, these are the spectral hounds of Annwn (the otherworld).
In Wales, they were associated with migrating geese, supposedly because their honking in the night is reminiscent of barking dogs.
Hunting grounds for the Cŵn Annwn are said to include the mountain of Cadair Idris, where it is believed “the howling of these huge dogs foretold death to anyone who heard them”.
According to Welsh folklore, their growling is loudest when they are at a distance, and as they draw nearer, it grows softer and softer. Their coming is generally seen as a death portent.
The Cŵn Annwn is associated with death, as it has red ears; the Celts associated the colour red with death. White is associated with the supernatural, and white animals are commonly owned by gods or other inhabitants of the Otherworld. Therefore, the Cŵn Annwn is associated with death and the supernatural.
They are supposed to hunt on specific nights (the eves of St. John, St. Martin, Saint Michael the Archangel, All Saints, Christmas, New Year, Saint Agnes, Saint David, and Good Friday, or just in the autumn and winter. Some say Arawn only hunts from Christmas to Twelfth Night. The Cŵn Annwn also came to be regarded as the escorts of souls on their journey to the the land of the dead. However, the Annwn of medieval Welsh tradition is an otherworldly paradise and nothing like Hell.
The hounds are sometimes accompanied by a fearsome hag called Mallt-y-Nos, “Matilda of the Night”. An alternative name in Welsh folklore is Cŵn Mamau (“Mother of the Hounds”).