Shub-Niggurath

Shub-Niggurath, often associated with the phrase "The Black Goat of the Woods with a Thousand Young", is a fictional deity in the Cthulhu Mythos of H. P. Lovecraft. The creature is sometimes referred to as "The Black Ram of The Forest With A Thousand Ewe", lending a male gender to the Great Old One that is often thought of as female. The being is first mentioned in Lovecraft's revision story "The Last Test" (1928); she is never actually described in Lovecraft's fiction, but is frequently mentioned or called upon in incantations. Shub-Niggurath is also referred to in the works of other Mythos authors, including August Derleth, Robert Bloch and Ramsey Campbell.

Inspiration
Robert M. Price points to a passage from "Idle Days on the Yann", by Lord Dunsany, one of Lovecraft's favorite writers, as the source for the name Shub-Niggurath:

And I too felt that I would pray. Yet I liked not to pray to a jealous God there where the frail affectionate gods whom the heathen love were being humbly invoked; so I bethought me, instead, of Sheol Nugganoth, whom the men of the jungle have long since deserted, who is now unworshipped and alone; and to him I prayed.[1] Notes Price: "The name already carried a whiff of sulfur: Sheol was the name for the Netherworld mentioned in the Bible and the Gilgamesh Epic."[2]

As for Shub-Niggurath's association with the symbol of the goat, Price writes,

we may believe that here Lovecraft was inspired by the traditional Christian depiction of the Baphomet Goat, an image of Satan harking back to the pre-Christian woodland deity Pan, he of the goatish horns and shanks. The Satanic goat is a device of much spectral fiction, as when in Dennis Wheatley's The Devil Rides Out the Archfiend's epiphany takes goat-headed form.