Rhiannon was a featured character in the Celtic mythology of Earth and was depicted in a literary epic known as the Mabinogi. She was a fearless, clever woman who possessed a beautiful, pale, swift horse. She was also an Other-worlder.
In the 24th century, Deanna Troi read the Mabinogi when she was growing up and was familiar with the Welsh character. Troi considered Rhiannon to be a “beautiful grown woman, almost a goddess, who had a special affinity for horses.”
In 2366, Troi had a waking dream in which the mythical Rhiannon had come to life and joined and a band of Other-worlders in an assault on CephCom, the headquarters of Cephalic Security in the city of Verity on the planet Rampart. In Troi’s dream, the 12-year-old Rampartian and the mythical character had merged together into an adult whom Troi saw riding valiantly on her horse among the security officers and one-eye thought-police robots. (TNG novel: Gulliver's Fugitives)