A slime mould cell.
It’s basically an amoeba, so not that scary. One whale-sized blob of cytoplasm that can’t do very much.
BUT … every now and then, it’ll join forces with hundreds of thousands of its siblings. They gloop together to form one massive superorganism that’s now no longer the size of a whale; it’s the size of Mount Everest.
It then sets off looking for food and digesting everything in its path. It squelches over our farms and cities, digesting them all. The armed forces try to blow it up, but every time they do, it just divides and carries on as before, eating everything and getting bigger. It is unstoppable.
Oh, and it’s smart. Capable of navigation, learning, and memory, despite not having a brain or any sensory organs as we know them.
It then decides to stop. It produces an armada of spores that disperse on the wind and waters to cover the globe. They return to being a single, whale-sized blob.
Until its myriad brethren over the world decide to coalesce again. Because it’s lunchtime.