Wikipedia
Tylwyth Teg ( Middle Welsh for " Fair Family"; ) is the most usual term in Wales for the mythological creatures corresponding to the Irish Aos Sí, comparable to the fairy folk of English and continental folklore. Other names for them included Bendith y Mamau ("Blessing of the Mothers"), Gwyllion or Ellyllon.
They are described as fair-haired and covet golden-haired human children whom they kidnap, leaving changelings (or "crimbils") in their place. They dance and make fairy rings and they live underground or under the water. They bestow riches on those they favour but these gifts vanish if they are spoken of, and fairy maidens may become the wives of human men.
As the Bendith y Mamau they are sometimes described as stunted and ugly. They ride horses in fairy rades (processions) and visit houses where bowls of milk are customarily put out for them. A changeling story tells of a woman whose three-year-old son was stolen by the fairies and she was given a threefold instruction by a " cunning man" (magician) on how to get him back. She removed the top from a raw egg and began stirring the contents, and as the changeling watched her do this certain comments he made established his otherworldly identity. She then went to a crossroads at midnight during the full moon and observed a fairy rade in order to confirm that her son was with them. Lastly she obtained a black hen and without plucking it she roasted it over a wood fire until every feather dropped off. The changeling then disappeared and her son was returned to her.
Usage examples of "tylwyth teg".
Iago appeared, dancing in the ring with the Tylwyth Teg, and was promptly plucked forth.
It was established that the Tylwyth Teg obeyed him and carried his messages through the air on soundless wings.
You see, all good Welsh-men are dark, dark of hair and dark of eyes, and the only fair-skinned creatures in Wales, in the old days, were the Tylwyth Teg.
Anyone as fair as me must have something to do with the Tylwyth Teg .