I believe you are looking for a rotfifer. http://www.absoluteastronomy.com/topics/Rotifer
A protist is more like a germ. It is a microscopic organism.
An animal that can pull weight like a horse pulling a carriage.
Generic name for animal-like protists would be Protozoa.
microscopic plankton
spider
plankton
If it's alive, not microscopic, and not plant matter, it's an animal.
An animalcule is a minute or microscopic animal or protozoan, or a tiny animal, such as an insect or mouse.
Microscopic living cells are called Protists. They are divided into three categories: animal like (or protozoan), plant like (algae), and fungus like. Protists have a wide species range because they are pretty much just organisms that don't fit into the classification of animal, plant, or fungus.
Tiny microscopic water animals are called plankton. They play a key role in aquatic food chains and come in various forms, including phytoplankton (plant-like) and zooplankton (animal-like).
"Phytoplankton" refers specifically to microscopic plant-like organisms that live in aquatic environments and photosynthesize to produce energy. "Plankton" is a general term that encompasses a wider range of organisms, including phytoplankton, zooplankton (animal-like organisms), and bacterioplankton (bacterial organisms). Phytoplankton are a subset of plankton.
the largest is the blue whale, ironicaly it eats one of the smallest plankton, but the smallest is the microscopic round worm.