Mount Elaios is some thirty stades away from Phigalia [in Arkadia] and has a cave sacred to Demeter surnamed Melaina (Black). The Phigalians accept the account of the people of Thelpousa about the mating of Poseidon and Demeter, but they assert that Demeter gave birth, not to a horse, but to Despoine (the Mistress), as the Arkadians call her. Afterward, they say, angry with Poseidon and grieved at the rape of Persephone. She put on black apparel and shut herself up in this cavern for a long time. But when all the fruits of the earth were perishing, and the human race dying yet more through famine, no god, it seemed, knew where Demeter was in hiding, until Pan, they say, visited Arkadia. Roaming from mountain to mountain as he hunted, he came at last to Mount Elaios and spied Demeter, the state she was in, and the clothes she wore. So Zeus learnt this from Pan, and sent the Moirai (Fates) to Demeter, who listened to the Moirai and laid aside her wrath, moderating her grief as well. For these reasons, the Phigalians say, they concluded that this cavern was sacred to Demeter and set up in it a wooden image. The image, they say, was made after this fashion. It was seated on a rock, like to a woman in all respects save the head. She had the head and hair of a horse, and there grew out of her head images of serpents and other beasts. Her tunic reached right to her feet; on one of her hands was a dolphin, on the other a dove. Now why they had the image made after this fashion is plain to any intelligent man who is learned in traditions.
They say that they named her Melaina (Black) because the goddess had black apparel. They cannot relate either who made this wooden image or how it caught fire. But the old image was destroyed, and the Phigalians gave the goddess no fresh image, while they neglected, for the most part, her festivals and sacrifices until the barrenness fell on the land Then they went as suppliants to the Pythian priestess and received this response :
‘Azanian Arkadians, acorn-eaters, who dwell in Phigaleia, the cave that hid Deo, who bare a horse, you have come to learn a cure for grievous famine, who alone have twice been nomads, alone have twice lived on wild fruits. It was Deo who made you cease from pasturing, Deo who made you pasture again after being binders of corn and eaters of cakes, because she was deprived of privileges and ancient honors given by men of former times. And soon will she make you eat each other and feed on your children, unless you appease her anger with libations offered by all your people, and adorn with divine honors the nook of the cave.’
When the Phigalians heard the oracle that was brought back, they held Demeter in greater honor than before, and particularly they persuaded Onatas of Aigina, son of Mikon, to make them an image of Demeter at a price" - Pausanias, Description of Greece 8. 42. 1-13
My favourite part in this account recorded by Pausanias is the oracle's message to the Phigalians. To me, it's metal as hell.
This isn't an attempt to recreate the wooden cult statue. I just wanted to draw the mare headed Deo. I imagine her with a white mare head, white like barley seed, while Melaina ("Black") refers to the dark cloak she wore in her grief and anger like the barley seed kept hidden beneath the black earth.