The last boss of Quake 1 is intended to literally be the Shub-Niggurath from the works of H.P. Lovecraft (a sci-fi alien take of it in any case). Shul-Nagruth's name is likely based on Shub-Niggurath, just as C'Thun is based on Cthulhu or Yogg-Saron is based on Yog-Sothoth from Lovecraft's works.