You could evolve a multicellular creature into a copepod or plankton-like organism. Either It could be in multicellular or in its own stage altogether.
I'm pretty sure this is the essence of the late Multicellular Stage. There, the player changes from a giant amongst cells to a minuscule creature in a 3D environment. A plankton is an organism that lives its life in the ocean currents, so any creature in Thrive that does the same could be considered such.