Gol-goroth (The Forgotten Old One) appears as a titanic, black toad-like monstrosity with a impossibly malevolent glare, or a tentacle covered, scaly, bat-winged entity. However, its physical form may alter to fit the worst nightmares of whoever is viewing it (spider-like to an arachniphobic person, etc.).
A dark god of Bal-Sagoth and Hyboria, with some modern cults centring around Britain and named as the god of the Black Stone in Hungary. It probably was worshipped in Atlantis under the name Golgor and also by the Little People via the Black Stones. Mentioned in the Book of Eibon as dwelling under Mount Antarktos, at the South Pole, it is also detailed in the Nameless Cults.
Gol-goroth was created by Robert E. Howard, and incorporated into the Mythos as a Great Old One by Lin Carter. However in the Robert E. Howard Solomon Kane story, "The Moon of Skulls" a deity named Golgor (probably same as Gol-goroth) is mentioned in connection with other Atlantean deities Valka, Hotah and Honen who also feature in the King Kull stories and therefore had a tenuous connection to the Cthulhu Mythos already.
A mistake by Lin Carter confused Gol-goroth with Groth-golka in the first printing of “The Fishers from Outside", but this has been rectified.