If you're seriously ill and start seeing animals that aren't really there, you're experiencing a delusion called zoopsia. When you're recovering from the flu and think you see a herd of pink and purple zebras in your bedroom, it's probably just zoopsia.
Zoopsia, an unusual type of hallucination, can be terrifying. Imagine witnessing flocks of pigeons swooping down from the ceiling, or a roaring tiger at the foot of your bed. Patients who are going through alcohol withdrawal sometimes experience zoopsia, often believing they see swarms of small rodents or insects. The word comes from the Greek zoion, "an animal."