A pet is an animal kept for companionship and enjoyment, as opposed to livestock, laboratory animals, working animals or sport animals, etc which are kept for economic reasons.

The most popular pets are noted for their loyal or playful characteristics, for their attractive appearance, for their song. Pets generally seem to provide their owners with non-trivial health benefits; keeping pets has been shown to help remove stress. [citation needed] There is now a medically approved class of 'therapy animals', mostly dogs, who are brought to visit confined humans (some illnesses, age related disabilities, etc). Walking a dog can also provide the animal and its owner with exercise, fresh air, and social interaction.

While in theory any animal might be a pet, in practice, only a small number of species of mammals, especially dogs and cats, and other small animals, such as birds, are practical for several reasons.

Fish have joined them more recently. Aside from the obvious, such as large animals not being able to fit inside small dwellings, which species are suited for being pets is less easy to understand.

One answer seems to be that a pet must either be so small or easily controlled that its own behavioral tendencies are irrelevant, or the animal must be actually domesticable.

Examples of the former are such things as fish (small ones; including carnivorous ones such as piranha), invertebrates or small reptiles.