Many varieties of giant fish have been spotted both above and under the ground, including many vicious flesh-eating ones. The larger ones are easily able to swallow a man whole.

Both the Frozen River (as it spills through the Valley of the Sparrows) as well as Whispering Lake are infested by schools of piranhas.

IMAGE01: Flesh-Eating Fish

SOURCE(S): Sorcerer, Zork: Conquest of Quendor, Return to Zork (design documents), and possibly other sources