Animals are a major group of organisms, classified as the kingdom Animalia or Metazoa. In general they are multi­cellular, responsive to their environment, and feed by consuming other organisms or parts of them. Their body plan becomes fixed as they develop, usually early on in their development as embryos, although some undergo a process of metamorphosis later on.

The word “animal” comes from the Latin word animal, of which animalia is the plural, and is derived from anima, meaning vital breath or soul. In everyday colloquial usage, the word usually refers to non-human animals. The biological definition of the word refers to all members of the Kingdom Animalia. Therefore, when the word “animal” is used in a biological context, humans are included.

A pet is an animal kept for companionship and enjoyment, as opposed to livestock, laboratory animals, working animals or sport animals, which are kept for economic reasons. The most popular pets are noted for their loyal or playful characteristics, for their attractive appearance, or for their song. Pets also generally seem to provide their owners with non-trivial health benefits; keeping pets has been shown to help relieve stress. There is now a medically-approved class of “therapy animals,” mostly dogs, who are brought to visit confined humans. Walking a dog can provide both the owner and the dog with exercise, fresh air, and social interaction.

Fish are aquatic vertebrates that are typically cold-blooded; covered with scales, and equipped with two sets of paired fins and several unpaired fins. Fish are abundant in the sea and in fresh water, with species being known from mountain streams (e.g., char and gudgeon) as well as in the deepest depths of the ocean (e.g., gulpers and anglerfish). They are of tremendous importance as food for people around the world, either collected from the wild (see fishing) or farmed in much the same way as cattle or chickens (see aquaculture). Fish are also exploited for recreation, through angling and fishkeeping, and fish are commonly exhibited in public aquaria as tropical aquarium fish. Fish have an important role in many cultures through the ages, ranging as wide as deities and religious symbols to subjects of books and popular movies.

The dog (Canis lupus familiaris) is a domestic subspecies of the wolf, a mammal of the Canidae family of the order Carnivora. The term encompasses both feral and pet variants. It is also sometimes used to describe wild canids of other subspecies or species.

Over time, the dog has developed into hundreds of breeds with a great degree of variation. For example, heights at the withers range from just a few inches (such as the Chihuahua) to roughly three feet (such as the Irish Wolfhound); colors vary from white through grays (usually called blue) to black, and browns from light (tan) to dark (”red” or “chocolate”) in a tremendous variation of patterns; and coats can be anything from very short to several centimeters long, from coarse hair to something akin to wool, straight or curly, or smooth.

The cat (or domestic cat, house cat) is a small carnivorous mammal that is often valued by humans for its companionship and its ability to destroy vermin. It has been associated with humans for at least 9,500 years. A skilled predator, the cat is known to hunt over 1,000 species for food. The cat is intelligent and can be trained to obey simple commands. Individual cats have also been known to learn to manipulate simple mechanisms (see cat intelligence). Cats use a variety of vocalizations and types of body language for communication, including mewing (”meow” or “miaow”), purring, hissing, growling, squeaking, chirping, clicking, and grunting.

Birds (class Aves) are bipedal, warm-blooded, egg-laying vertebrate animals. Birds evolved from theropod dinosaurs during the Jurassic, and the earliest known bird is the Late Jurassic Archaeopteryx. Ranging in size from tiny hummingbirds to the huge Ostrich and Emu, there are between 9,000-10,000 known living bird species in the world, making them the most diverse class of terrestrial vertebrates.

Modern birds are characterised by feathers, a beak with no teeth, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a light but strong skeleton. Most birds have forelimbs modified as wings and can fly, though the ratites and several others, particularly endemic island species, have also lost the ability to fly.

Many species of bird undertake long distance annual migrations, and many more perform shorter more irregular movements. Birds are social and communicate using visual signals and through calls and bird song, and participate in social behaviours including cooperative hunting and breeding, flocking. Birds are primarily socially monogamous, with engagement in extra-pair copulations being common in some species; other species have polygamous or polyandrous breeding systems. Eggs are usually laid in a nest and incubated and most birds have an extended period of parental care after hatching.

Humans make use of birds in a number of ways: many are important sources of food, acquired either through hunting or farming, often commercially. Some species, particularly songbirds and parrots, are popular as pets. Birds figure prominently in all aspects of human culture from religion to poetry and popular music. About 120–130 species have become extinct as a result of human activity since 1600, and hundreds more prior to this. Currently, numerous species of birds are threatened with extinction by human activities and efforts are underway to protect them.

Reptiles are tetrapods and amniotes, animals whose embryos are surrounded by an amniotic membrane, and members of the class Sauropsida. Today they are represented by four surviving orders: Crocodilia (crocodiles, gharials, caimans and alligators): 23 species, Sphenodontia (tuataras from New Zealand): 2 species, Squamata (lizards, snakes and amphisbaenids (”worm-lizards”)): approximately 7,900 species, Testudines (turtles and tortoises): approximately 300 species.

Modern reptiles inhabit every continent except for Antarctica, although their main distribution comprises the tropics and subtropics. Though all cellular metabolism produces some heat, most modern species of reptiles do not generate enough to maintain a constant body temperature and are thus referred to as “cold-blooded” or ectothermic (the Leatherback Sea Turtle might be an exception, see also gigantothermy). Instead, they rely on gathering and losing heat from the environment to regulate their internal temperature, e.g, by moving between sun and shade, or by preferential circulation — moving warmed blood into the body core, while pushing cool blood to the periphery. In their natural habitats, most species are adept at this, and can usually maintain core body temperatures within a fairly narrow range. Reptiles are thick-skinned; unlike amphibians, they do not need to absorb water. While this lack of adequate internal heating imposes costs relative to temperature regulation through behavior, it also provides a large benefit by allowing reptiles to survive on much less food than comparably-sized mammals and birds, who burn much of their food for warmth. While warm-blooded animals move faster in general, an attacking lizard, snake or crocodile moves very quickly.

The horse (Equus caballus, sometimes seen as a subspecies of the Wild Horse, Equus ferus caballus) is a large odd-toed ungulate mammal, one of ten modern species of the genus Equus. Horses have long been among the most economically important domesticated animals; although their importance has declined with mechanization, they are still found worldwide, fitting into human lives in various ways. The horse is prominent in religion, mythology, and art; it has played an important role in transportation, agriculture, and war; it has additionally served as a source of food, fuel, and clothing.

Rabbits are small mammals in the family Leporidae of the order Lagomorpha, found in several parts of the world. There are seven different genera in the family classified as rabbits, including the European rabbit (Oryctolagus cuniculus), cottontail rabbit (genus Sylvilagus; 13 species), and the Amami rabbit (Pentalagus furnessi, an endangered species on Amami Oshima, Japan). There are many other species of rabbit, and these, along with cottontails, pikas, and hares, make up the order Lagomorpha. Rabbits generally live between four and 20 years. A rabbit’s gestation period is 31 days.

Hamsters are rodents belonging to the subfamily Cricetinae. The subfamily contains about 18 species, classified in six or seven genera. Because they are easy to breed in captivity, hamsters are often used as lab animals and kept as pets.