Sponge

Sponge is a blue creature type. It has only been used on <c>Walking Sponge</c> from Urza's Legacy and <c>Thought Sponge</c> from Commander 2019. 
Sponges are aquatic, chiefly marine animals, which have a porous structure and usually a horny, siliceous or calcareous internal skeleton. They occur in large, sessile colonies. 
Though scientifically speaking there is no such thing as a walking sponge, both Magic: The Gathering sponges are shown to be mobile.