Most of the Welsh surnames of today were not originally surnames at all. The Welsh were "patronymic", taking the given name of the father as one's surname, so in essence, the surname changed with each generation. Hugh's son William would be William Hughes, and his son Thomas would be Thomas Williams, and his son David would be David Thomas, etc. Surnames started to "stick", in general, around the late 18th century. A better explanation can be found at the GENUKI (Genealogy for the United Kindom and Ireland) website, on the pages for Wales. It's just my opinion, but I would think THOMAS was and still is a popular given name (taken from the Bible, Jesus' disciple) that through patronymics, became a common surname among the Welsh.