Unlike the largest animal, the "smallest animal" is a title which has not been fully defined, as it is both difficult to identify the size of a microscopic organism compared to another organism. I would say though, Myxozoa is probably one of the closest answers you can get. These microscopic Cnidarians are parasites of fish and were once thought to be Protozoa. The smallest known species are under a hundredth of a millimeter. Another possible candidate would be the Rotifer, while they are 5 times larger than Myxozoans , they are non-parasitic and often free-swimming.