Sponge

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.

Although some cartoons would have you believe different [1], there's no such thing as a walking sponge. Going by the picture, by rights the subtype of Walking Sponge should have included Crab. Its abilities have nothing to do with the subtype.