Originally created by VersaLife before the Great Collapse to serve as a guard animal, the greasels proved to be very difficult to control, and when the Collapse hit and wiped out the security systems at VersaLife's various facilities, they escaped along with the grays and the karkians.
Resembling dinosaurs, greasels are vicious predators with keen senses and the ability to expectorate a highly toxic venom. They mainly eat small live mammals, particularly cats, but they aren't picky; the unlucky scientists who became trapped in JC Denton's Sanctuary learned this the hard way, as a pack of escaped greasels from the nearby VersaLife labs slowly hunted them until none of the scientists remained alive. As seen in Antarctica, the creatures exhibit the ability to survive in very cold temperatures.
In spite of the danger greasels pose, they are sought after by certain segments of post-Collapse society (the street value as of 2072 is around 375 credits.) In Lower Seattle, greasel fights have become a popular attraction at the "Greasel Pit Bar", where an intrepid trainer (or a smart gambler) can make considerable amounts of money with the right greasel. The current champion is Gobzilla, whose trainer is Tina.
It is implied by some conversations in the games that greasels taste good like chicken when cooked.
- The Red Greasel Hunter is a unique pistol in Deus Ex: Invisible War specially modified by the Omar to hunt greasels.