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.
Did you know? The production name of the Banshee Grendels was the Brigand Grendels, and the two that beta-tester TreeSprite received had warped from a world named Brigand's Outpost.