In Creatures, The Grendel Mother is only a warty tube high up in the branches of the huge tree some call Yggsdrasil. She is completely inactive until she senses the absence of a Grendel in the game, upon which she will lay a single egg. Unless there is interference by the player, there is only one Grendel in the game at a time, always male, and always infertile.
Did You Know? In C1, if you rename any valid genome to 'gren.gen', the Grendel Mother's children will use that genome! However, if you use a norn genome, this can cause overpopulation rather quickly, as the Grendel Mother still senses that there are no grendels in the world.
Creatures 2 saw the seeming disappearance of the Grendel Mother. Instead, a small sub-sea level room in the Volcano holds a large number of Grendel eggs which mysteriously appear as others hatch. Like C1, there is only one male, and though it's possible to create a female, Grendel couples were always infertile.
Creatures 3's Grendel Mother (seen below) was more of a machine, located in the Jungle Terrarium. This time around however, two male Grendels would be hatched and maintained. Also, in splicing the two provided, the player could create a female that could actually breed (although giving birth will age normal female Grendels prematurely).
Have you been having trouble with norn or ettin eggs being laid by the Grendel Mother? Don has made several patches to fix this for various norn breeds - get them here, at WebPetz! An alternative fix is the Grendel and Ettin Maker Update, which also makes it possible for females to hatch from Grendel Mother eggs.
There is also a website called The Grendel Mother.