Parasite (All Tomorrows)

The Parasites were a posthuman species created by the Qu from a colony of the Star People. They were only one line of human species on their homeworld, the other being ape-like primitive creatures.

There were many species of parasitic humans on this world, ranging from tortoise-like bloodsuckers to fist-sized creatures living attached to their hosts. Their hosts were always humans of the ape-like linage.

For forty million years, the Qu overlooked the world, guaranteeing the survival of the parasites so that they could punish the other humans for fighting back against the Qu-invasion.

After the departure of the Qu, many parasites died out because their hosts could drown, burn or eat them. One species sterilized its hosts by living in their vaginas, dooming the hosts and themselves.

Only few species survived by regulating their dumb hosts, not killing them by over-infestation and thus ensuring their own long-term survival as well. Eventually, they evolved into the Symbiotes.