Arion

Poseidon and Demeter coupled in the form of a horse. (Whether this was consensual on Demeter’s part depends on the version of the myth.) From the union, Demeter birthed twins, Despoena and Arion. Arion is a beautiful oracular, immortal talking stallion. Invoke him for information. Some people speculate that vestiges of his veneration survive in the talking horse head of the Brothers Grimm fairy tale The Goose Girl.

John and Caitlin Matthews, The Element Encyclopaedia of Magical Creatures, 2013, Arion, p 33