Sponge

Sponge is a blue creature type. It has only been used on Walking Sponge from Urza's Legacy and Thought Sponge 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.