I know. This is probably the weirdest fairy story on TMA now. But it was good enough for Wirt Sikes and his British Goblins, so it's good enough for me. The Coblynau are Welsh mine fairies, so maybe they're what William Evans saw. Maybe.
W.E. of Hafodafel, going a journey upon the Beacon Mountain, very early in the morning, passed by the perfect likeness of a Coal Race, where really there was none; there he saw many people busy; - some cutting the coal - some carrying it to fill the sacks - some rising the loads upon the horses' backs, &c. - This was an Agency of the Fairies upon his visive faculty, and it was a wonderful extra natural thing, and made a considerable impression upon his mind. He was of undoubted veracity, - a great man in the world, - and above telling an untruth. The power of Spirits, both good and bad, is very great, not having the weight of bodies to in-cumber and hinder their agility.