In Wales, the fairy colours are white and gold, making fairies seem akin to beings of light. However, light doesn't always mean good. Hafgan ("Summer White") battles the hero Pwyll while Gwyn ap Nudd ("White, son of Fog") tempts saints and drives people insane.
John Duncan

The most potent magic of the great Welsh magician Gwydion was turning plants into people. He worked with his uncle Math to transform a bunch of flowers into a wife for his ward Lleu Llaw Gyffes, and later turned a grove of trees into an army to fight an invading fairy host.
Dreoilin

In Welsh legend, the Black Wyrm of the Barrow was a huge serpent that lived in a barrow. Since a barrow is a pagan tomb, some scholars have assumed that the Wyrm was inspired by a pagan god. It was slain by Peredur (Sir Percival), one of King Arthur's most Christian warriors.
Alan Lee

The British King Gwenddoleu ap Ceidio was said to possess a magic gold & silver chess board whose pieces move by themselves and which could play any opponent on its own. This board appears in many King Arthur legends, and it's one of the 13 Treasures of Britain guarded by Merlin.

The adar llwch gwin were huge birds (or possibly griffins) in Welsh myth. Drudwas ap Tryffin owned a pair. Just before he dueled King Arthur, Drudwas ordered them to kill the first man to enter the battlefield. However, Drudwas accidentally entered first, so his birds tore him to pieces.

In medieval Welsh legends, fairies look human, and aren't treated as nonhuman by human characters or collectively referred to by any general term (such as "fairy" or "Tylwyth Teg"). It's just implied that maybe these mysterious people with inexplicable magic aren't quite normal.
Alan Lee

In Welsh myths, Merlin buried a great treasure in a golden cauldron inside the Welsh hill Dinas Emrys - where King Vortigern once tried to sacrifice him to build a tower. Only Merlin's heir (who must have blond hair) will be able to find the hidden cavern and claim the treasure.

In Welsh mythology, the magical white pig Henwen gave birth to a kitten who grew to gigantic size and cruelty. The Cat of Palug slaughtered 180 warriors on the Isle of Anglesey before it was slain by King Arthur's brother Kay (or by Arthur himself in later versions).
Peter Diamond

In Welsh folklore, Bedwyr (Bedivere) is one of King Arthur's two best friends, and one of his best warriors (despite being one-handed). Bedwyr lost his hand battling Gwrgi Rough-Grey, a dog-headed man (strangely reminiscent of how the Norse god Tyr lost his hand).
Margaret Jones

Though many people claim that King Arthur sleeps in Avalon, others theorize that Arthur and his warriors sleep in a cavern deep underground, or that Arthur's ghost leads the Wild Hunt across the sky, or that he has been reincarnated as a raven watching Britain.

In Welsh legends, Peredur (Percival) is the most Christian of King Arthur's knights. He once battled a man in rusty armour who rode a bony horse and came from under a stone slab. It's implied that this stone slab is some ancient tomb, making this man some kind of pagan undead.

The top of Cadair Idris, a mountain in Wales, is often shrouded by mist. In Welsh folklore, the fairy huntsman Gwyn ap Nudd uses this mist as a doorway from the Otherworld to Earth. When the mist is heavy, he and his hounds prowl Cadair Idris looking for prey.
Matthew Austern

In Welsh legends, the Cwn Annwn ("Hounds of the Otherworld") are white fairy dogs that race across the sky as the Wild Hunt. Their baying can be mistaken for the honking of geese, though it actually gets quieter the closer the Cwn Annwn get to their prey.
Roger Garland

Every May 1 until the coming of Judgement Day, Welsh hero Gwythyr battles Gwyn ap Nudd (king of the dead) for Creiddylad (Gwythyr's kidnapped fiancée). As May 1 is the middle of Spring and Gwyn is a death figure, many scholars interpret this as the war between Winter and Summer.

Though King Arthur's most famous magical item is his sword, Welsh legends give him a full arsenal forged either by God or Avalon's fairies. They include his spear Rhongomyniad, his dagger Carnwennan, his shield Wynebgwrthucher, and his cloak of invisibility Gwenn.
Charles Ernest Butler

The one unicorn to appear in a medieval Welsh legend was a surprisingly savage beast that resembled a deer with one huge antler on its forehead. It slaughtered any human that entered its forest and would suck lakes dry with a single gulp, so all the fish died.

In Welsh mythology, an army of fairies invaded northern Wales. This army included such monsters as a giant toad with 100 claws, a giant serpent with 100 damned souls trapped in its body, and a 100-headed beast so huge that battles were fought on its tongues.
Jason Kang

Five of the Thirteen Treasures of the Island of Britain in Welsh folklore are tied to food: a basket of infinite food, a horn that holds any drink desired, a cauldron that never boils for a coward, a knife that serves twenty-four people at dinner, and a crock & dish that can contain any food.

In Welsh legend, the Awen is the divine force that possesses poets and bards to inspire their art, and possesses seers to give them visions of the future. However, if someone who lacks the proper will is possessed, the Awen can drive them insane.
John Hall

In the Welsh legend of Sir Geraint, he is forced to abandon the life he loved as a knight at King Arthur's court to take over the tedious rulership of his father's kingdom. This drags Sir Geraint into a deep depression, and eventually he becomes suicidal.
Gustave Dore