Free Will Versus Parasites
The lancet fluke changes an ant's behavior so that it grabs ahold of grass blades to get eaten by cattle or sheep -- but only at night. When the morning comes, the ant returns to its normal day-to-day life. (Have you been feeling inexplicably tired, lately?)
Leucochloridium paradoxum makes snails climb up into the light -- to be eaten by birds.
Nematomorph hairworm makes grasshoppers go jump in a lake.
The Hymenoepimecis wasp injects larva into spiders, and then the larve make the spider weave a cocoon for them, instead of its usual webs.
The Ampulex compressa wasp makes a cockroach into its obedient zombie slave and then leads it to its doom.
Toxoplasma gondii in cat feces make rats unafraid of cats and affects human behavior too, possibly on a massive (billions of people), culture-changing scale.
Ceti eels enter humans through the ear and wrap themselves around the cerebral cortex, making the victim extremely susceptible to suggestion. Later as they grow, follows madness and death. They can be used as pets by eugenically-bred leaders, though they're still not quite domesticated.