In Welsh mythology, Rhiannon is a horse goddess depicted in the Mabinogion. She is similar in many aspects to the Gaulish Epona, and later evolved into a goddess of sovereignty who protected the king from treachery.
Rhiannon was married to Pwyll, the Lord of Dyfed. When Pwyll first saw her, she appeared as a golden goddess upon a magnificent white horse. Rhiannon managed to outrun Pwyll for three days, and then allowed him to catch up, at which point she told him she'd be happy to marry him, because it would keep her from marrying Gwawl, who had tricked her into an engagement. Rhiannon and Pwyll conspired together to fool Gwawl in return, and thus Pwyll won her as his bride. Most of the conspiring was likely Rhiannon's, as Pwyll didn't appear to be the cleverest of men. In the Mabinogion, Rhiannon says of her husband, "Never was there a man who made feebler use of his wits." After Pwyll's death, Rhiannon married Manawyden.
Her name, Rhiannon, derives from a Proto-Celtic root which means "great queen," and by taking a man as her spouse, she grants him sovereignty as king of the land. In addition, Rhiannon possesses a set of magical birds, who can soothe the living into a deep slumber, or wake the dead from their eternal sleep.
Primarily, though, Rhiannon is associated with the horse, which appears prominently in much of Welsh and Irish mythology. Many parts of the Celtic world -- Gaul in particular -- used horses in warfare, and so it is no surprise that these animals turn up in the myths and legends or Ireland and Wales. Scholars have learned that horse racing was a popular sport, especially at fairs and gatherings, and for centuries Ireland has been known as the center of horse breeding and training.