Alternatively spelled Rhidian from the 20th century, Rhydian (RID-ee-an) is a Welsh boy's name meaning "red," from the element rhudd (rudd), sharing some similarities with Rowan. An early Welsh saint may have had this name. There is a church established by the saint in the 6th century
Namesakes include British-Taiwanese actor Rhydian Vaughan (pictured above), Welsh singer Rhydian Roberts, novelist Rhidian Brook (who coincidentally wrote The Testimony of Taliesin Jones - I just wrote about Taliesin), and bass player Rhydian Dafydd for the band The Joy Formidable.
As a name in the U.S., Rhydian has not been used more than four times in a year, and is therefore not recorded by the SSA. White Pages tells me that only one exists.