From Creatures Wikia
According to The Easter Egg Archive, Easter Eggs are "any hidden, entertaining thing that a creator hides in their creation only for their own personal reasons." There are many such things to be found within the Creatures series, both in and out of the games. Some of the more renowned ones include Borland, Len the Pen, and Grendel Invaders.
A complete list of known Easter Eggs can be found here.