Giant Oyster
This monster looks much like a regular oyster, except that it is 6 feet in diameter, and stands to a height of 4 feet. Its Armor Class is 5 when open, -2 when closed. When attacked from a distance, a giant oyster closes its shell anddoes not open for at least 1 turn.
Giant oysters may be found in nearly any type of water surrounding, though they are most often found in cold, shallow water.