From Creatures Wikia
The Bulbous Grendels lived in the old volcano and stood virtually no chance of survival. To the left there was an airlock for the sub, which the grendels could walk into and drown themselves. To the right there was a long drop into the volcano, which was highly radioactive and slowly destroyed their bone marrow. If they got out of that there was a 'swamp' that had many poisonous Deathcap Mushrooms.
When breeding norns in Creatures 2, the player rarely encounters Grendels. Contrary to their description, Bulbous Grendels do get reward for hitting another creature, but have no instinct to do so in the first place.