From Creatures Wikia
The Grendel from Creatures 3 has definitely changed compared to their predecessors - as a breed, they are more vicious and likely to steal norn eggs. They take the norn eggs to the Jungle Terrarium, where the grendels also spend most of their time. Most grendels come from the Grendel Mother which can only lay male eggs. Generally, the Grendel Mother waits several minutes before laying an egg.This breed can live with Norns happily but both must be introduced at the baby stage.