The gwyllgi (Welsh pronunciation: [ˈɡwɪɬɡi]; compound noun of either gwyllt "wild" or gwyll "twilight" + ci "dog"[1]) 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.
[2] It is the Welsh incarnation of the black dog figure of English folklore.
[4] In Blood of the Earth, by Faith Hunter, many members of the local cult-like church have been turned into gwyllgi and capture the main character, Nell.
In the Well of Shadows series by SL Harby, gwyllgi play a prominent part in the story of the Songspear, Aristoi.
In The Beginner's Guide to Necromancy series by Hailey Edwards, a gwyllgi named Hood is one of the watchmen who guards the Faraday, a high-end condominium where Linus lives.