Oyster is a blue creature type that is only used once, for the card Giant Oyster from Homelands. The card reappeared in the Time Spiral/Timeshifted set. The Giant Oyster keeps target creature tapped and puts -1/-1 counters on that creature.
An oyster is an edible, marine, bivalve mollusk with an irregularly shaped calicified shell, occurring on the bottom or adhering to rocks or other objects in shallow water. Some oysters produce pearls.