From Creatures Wikia
The Banshee Grendels are a breed created by Creature Labs which finishes off the Docking Station storyline. You could only buy the Banshee Grendel Pack at the Gameware Creatures Mall. As of 2013, Banshee Grendels, along with other Mall Breeds are unavailable, with no indication when they will be back. The Banshee Grendel Story is extra-long and explains both their name and source.
The Banshee Grendel Pack comes with a Banshee Egg Stone, the Mecha Grendel Toy, and improved voices for female Grendels.
Did you know? The production name of the Banshee Grendels was the Brigand Grendels, and the two that beta-tester TreeSprite recieved had warped from a world named Brigand's Outpost.