Giant slugs are essentially immense versions of the tiny garden slugs we all know and love. The lairs of these creatures can easily be detected by the trails of ooze that is deposited in its wake. These slugs defend themselves by spraying their attacker with vile ichors. Slugs are notoriously sensitive to a certain common substance; that substance is salt.
One giant slug encountered in the ruins of a lighthouse near Grubbo-by-the-Sea (defeated in 966 GUE) was as long as a human is tall. Other enormous slug-shaped creatures, a bloit long, were seen on the surface of the planet below Zork, engaged in fierce combat and able to instantly devour a human with ease.

During the Great Monster Uprising of the Second Age of Magic, giant slugs were reported in the following regions: White Cliffs BeachFenshire, Flathead Fjord, Mirror LakeTidal Flats, Miznia Marshlands, Miznia JungleCrab Island, Largoneth Castle, and the G.U.E. Tech Training Grounds in the Ethereal Plane of Atrii.


SOURCE(S): Beyond Zork, Zork Zero, Legends of Zork, Zork:Nemesis (design documents)