From Creatures Wikia
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 abscence 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 dissappearance 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).
There is also a website called The Grendel Mother.