In Celtic myth, the horse goddess Epona (‘Pony’) carries a key which unlocked both the stable door and the gates of the Otherworld. She rides between the worlds at will, to carry souls to the afterlife on horseback, accompanied by blackbirds and larks. She is also a shamanic deity, who carries the seer or bard to the Otherworld. She is, in fact, the Fairy Queen who travels between the realms on her white horse. In Wales, she was called Rhiannon, whose seven blackbirds called dreamers into Fairyland.
Franklin, Anna. Path of the Shaman (Eight Paths of Magic Book 2) . Lear Books. Kindle Edition.