The gwyllgi (Welsh pronunciation: [ˈɡwɪɬɡi]; 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 with baleful breath and blazing red eyes.
It is often referred to as "The Dog of Darkness" or "The Black Hound of Destiny", the apparition's favourite haunt being lonely roads at night.
- Geiriadur Prifysgol Cymru, vol. 2, p. 1765.
- Dogs of Darkness, BBC Wales