A pet or companion animal is an animal kept primarily for a person's company, protection, or entertainment rather than as a working animal, livestock, or laboratory animal. Popular pets are often noted for their attractive appearances, intelligence, and relatable personalities. Two of the most popular pets are dogs and cats. Other animals commonly kept include: rabbits, ferrets, pigs; rodents, such as gerbils, hamsters, chinchillas, rats, and guinea pigs; avian pets, such as parrots, passerines, and fowl; reptile pets, such as turtles, lizards and snakes; aquatic pets, such as fish, freshwater and saltwater snails, and frogs; and arthropod pets, such as tarantulas and hermit crabs. Small pets may be grouped together as pocket pets, while the equine and bovine group include the largest companion animals. Pets provide their owners (or "guardians") both physical and emotional benefits. Walking a dog can provide both the human and the dog with exercise, fresh air, and social interaction. Pets can give companionship to people who are living alone or elderly adults who do not have adequate social interaction with other people. There is a medically approved class of therapy animals, mostly dogs or cats, that are brought to visit confined humans, such as children in hospitals or elders in nursing homes. Pet therapy utilizes trained animals and handlers to achieve specific physical, social, cognitive or emotional goals with patients. Some scholars, ethicists and animal rights organizations have raised concerns over keeping pets because of the lack of autonomy and objectification of nonhuman animals..