Any animal kept by humans for companionship or pleasure rather than for utility. The main distinction between pets and domesticated livestock is the degree of contact between owner and animal. Another distinction is the owner's affection for the animal, which is often returned. Dogs are known to have been kept as pets since prehistoric times; cats, since the 16th century BC; and horses, since at least 2000 BC. Other common pets include birds, rabbits, rodents, raccoons, reptiles, amphibians, and even insects. The trend toward making pets of exotic animals (e.g., monkeys and ocelots) is worrisome because owners can rarely provide for their needs, and the animals' already precarious populations are further depleted when members are sold for pets.