Unusual Animal Friends

Other model organisms belonging to the animal kingdom include the mouse (Mus musculus) and zebrafish (Danio rerio).
History of classification
Aristotle divided the living world between animals and plants, and this was followed by Carolus Linnaeus (Carl von Linné), in the first hierarchical classification. Since then biologists have begun emphasizing evolutionary relationships, and so these groups have been restricted somewhat. For instance, microscopic protozoa were originally considered animals because they move, but are now treated separately.