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.