The derivation of Rhiannon from *Rigantona would mean “Great or Divine Queen” and Rhiannon was a queen. Likewise, Teyrnon could derive from *Tegernonos, “Great King”. There also seems to be a plausible connection with the Madron-Mabon legend too, and horses seem definitely to play a part. Epona means the “Great Mare”. I think would could be dealing with epithets again. What do you think?