Rhiannon comes from the Welsh — from Rigantona, meaning "great queen" or "divine queen." In the medieval Welsh mythological cycle The Mabinogion, Rhiannon is one of the central figures: a queen of the Otherworld who appears on a magical white horse, the embodiment of grace under terrible injustice.
Today it remains in continuous use in Wales and is rising in the U.S.
Rhiannon does not currently appear in the US Social Security Administration's top 1,000 girls' names, so we don't publish a US rank or birth count for it. That says nothing about the name's standing elsewhere in the world — only that it sits outside the ranked US data we rely on.
In Pythagorean numerology the letters of Rhiannon reduce to 3, The Communicator. This is a traditional interpretive system, not a factual claim about the name.