In the myth of the isolated land of Arcadia, the river spirit of the underworld appears as a horse (Poseidon Hippios), as was usual in northern European folklore. He mates with the mare, Demeter, and from the union she bears the horse, Arion, and a daughter who originally had the shape of a mare too. It seems that the Greek deities started as powers of nature, and then they were given other attributes. These powers of nature developed into a belief in nymphs and in deities with human forms and the heads or tails of animals. Some of them, such as Pan and the Silenoi, survived into the classical age. The two great Arcadian goddesses, Demeter and Despoina (later Persephone), were closely related to the springs and the animals, and especially, to the goddess Artemis (Potnia Theron: "The mistress of the animals"), who was the first nymph.