According to my Welsh names reference book, the masculine name of "Terfel" is still in use in Wales today. It is a variation of the name "Derfel" or "Derfael", which was the name of an early saint from the Llandderfel chuch in Merionethshire. The name means either "Oak Prince" (derw + mael) or "Stubborn Prince" (der + mael).
-- Nanaea