Someone pointed out that scientists estimate there are only 100 million species on the earth, not six billion, like I claimed. However, they are neglecting some categories:
Colonies of bacteria that live in the clouds, move in swarms, and behave like slime-molds the size of islands.
Anemone-like creatures that exist within the earth's core. They have a chemistry based on exotic states of hot, compressed metals and reproduce by fission.
Memetic parasites which are more developed than mental viruses. Some have evolved a dormant state where they can live for hundreds of years in reading material. For example, Zoroastrianism or "Crimson and Clover."
Certain forms of currents in water are similar to smoke rings, but more stable and much more complex, and have evolved the ability to reproduce. They are completely invisible to humans, but well-known by dolphin and whale scientists.
Various uncatalogued fairy species that live a tribal lifestyle and hunt the mighty Tardigrade.