Gwyllion or gwyllon is a Welsh

Welsh language

Welsh is a member of the Brythonic branch of the Celtic languages spoken natively in Wales, by some along the Welsh border in England, and in Y Wladfa...

 word with a wide range of possible meanings including "ghosts, spirits," "night-wanderers (human or supernatural) up to no good, outlaws of the wild" etc. Gwyllion is only one of a number of words with these or similar meanings in Welsh. It is a comparatively recent word coined inadvertently in the 17th century by the Welsh lexicographer Dr John Davies (Mallwyd)

John Davies (Mallwyd)

Dr John Davies, Mallwyd was one of Wales's leading scholars of the late Renaissance. He wrote a Welsh grammar and dictionary. He was also a translator and editor and an ordained minister of the Church of England....

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An account

One account of a species of gwyllion refers to them as ugly female spirits that are usually described as wearing ash colored with an oblong four-pointed hat, and usually carrying a pot in one hand. They also are known by their disturbing laughter and their cries of "Wb!" . They are said to live in mountainous areas and love to mislead travelers or scare them. However, if one of these gwyllion in disguise enters your home and is treated right, it is believed they will do no harm. These gwyllion are supposedly repelled by metal knives, and a flash of a knife is thought to usually be enough to send them away.
Mountain Fey,Fae, or fairy.

See also

• Gwylliaid Cochion Mawddwy
  Red Bandits of Mawddwy
  The Red Bandits of Mawddwy were a band of red-haired robbers, highwaymen or footpads from the area of Mawddwy in Mid Wales in the 16th century, who became famous in folk literature....
  
  , the so-called "Red Bandits of Mawddwy"