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;
Greasels are typically found in MJ12 facilities, such as VersaLife and first seen in the Liberty Island MJ12 lab. Greasels can be found in Paris, which Tracer Tong suggests were either released by MJ12 or had escaped from a nearby facility. Escaped greasels can be found in the Pasadena Ocean Lab after the attack on the facility opened their cases and are also created in Universal Constructors in Area 51.
The caged greasels in the Liberty Island MJ12 lab can released via button or computer, causing them to attack all humans in the area, including the player. The greasels in VersaLife can be released, causing them to befriend the player and attack MJ12 in the area.
Unlike karkians, greasels will prioritize fighting over eating, if left alone they will eat any corpses nearby, restoring 2 health on munch.
Greasels move slowly in water, but their ranged spit still works underwater. Greasels also don't drown when swimming.
The unlucky scientists who became trapped in JC Denton's Sanctuary, encountered 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 350 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.
Some greasels are kept as pets, with training manuals wrote with instructions on how to care for them. Greasels are often kept in cages and fed cats when kept as pets.
It is implied by some conversations in the games that greasels taste good like chicken when cooked.
VersaLife later developed an additional variant, a red version of the greasel that is more deadly, killing rather than causing fainting.
|"We've discovered that one of VersaLife's final projects was the development of a meaner, tougher, red-colored breed of greasel.
Rather than natural evolution, it looks like the DNA was infused with new data from outside the existing green greasel genome.
This genetic manipulation has made the animals considerably more aggressive."
|— Red Greasels, Next Rev|
|"There's no stopping the red greasels though. Their attacks are too...effective. No feints and no play. One mistake and the trainer ends up dead."|
|— Red Greasels|
Red greasels are said to be more aggressive, more deadly, have better endurance and faster breeding than green greasels, with female greasels preferring red greasels over green.
- The Red Greasel Hunter is a unique pistol in Deus Ex: Invisible War specially modified by the Omar to hunt greasels.