Phasm

In the Dungeons and Dragons fantasy role-playing game, the Phasm is an Aberration. It resembles an amorphous blob of giant, brightly colored cells, organs, bacteria and ocean plants, roughly 5 feet in diameter and 2 feet tall, but can assume the form of almost any object or creature by developing a caste around these innards. It lives in organic, underground caves.

The Phasm, though classified as an Aberration, has similar habits to an Ooze when in normal form. When harrased or pursued, however, it morphs in the shape of and develops the caste of the deadliest creature it knows, and then attacks in whatever way it can in that form. When in natural form, it can attack with a pseudopod