Terrestrial animals are animals that live predominantly or entirely on land (e.g., cats, ants, emus), as compared with aquatic animals, which live predominantly or entirely in the water (e.g., fish, lobsters, octopuses), or amphibians, which rely on a combination of aquatic and terrestrial habitats (e.g., frogs). The term terrestrial is also frequently used for species that live primarily on the ground, in contrast to arboreal species, which live primarily in trees.
Terrestrial invasion is theorized to be one of the most important events in the history of life. Terrestrial lineages theoretically evolved in several animal phyla, among which vertebrates, arthropods, and mollusks are representatives of more successful groups of the epifaunal terrestrial life.
Terrestrial animals do not form a unified clade; rather, they share only the fact that they live on land. The transition from an aquatic to terrestrial life has theoretically evolved independently and successfully many times by various groups of animals, though this can't be proven through current scientific data. Most terrestrial lineages are theorized to have originated under mild or tropical climate during the Paleozoic and Mesozoic, whereas few animals became fully terrestrial during the Cenozoic.