Rhan-Tegoth is a Great Old One appearing in "The Horror in the Museum," a story H. P. Lovecraft ghostwrote for Hazel Heald in October 1932 and published in Weird Tales in July 1933.
The story is set in a private wax museum in Southwark, London, where a character named Rogers creates a wax statue of Rhan-Tegoth holding a mangled man. Lovecraft claimed substantial authorship of the tale in a letter to Clark Ashton Smith.