A Walking Sponge. Art by Ron Spencer.
Sponge is a blue creature type that is only used once, for the card Walking Sponge from Urza's Legacy.
Sponges are aquatic, chiefly marine animals, which have a porous structure and usually a horny, siliceous or calcareous internal skeleton. They occurre in large, sessile colonies.
Though scientifically speaking there is no such thing as a walking sponge, going by the picture it could be argued the subtype of Walking Sponge should have included Crab. Its abilities have nothing to do with the subtype, art, or flavor text.
