A pet, or companion animal, is an animal kept primarily for a persons company or entertainment rather than as a working animal, livestock, or a laboratory animal. Popular pets are often considered to have attractive appearances, intelligence, and relatable personalities, but some pets may be taken in on an altruistic basis and accepted by the owner regardless of these characteristics. 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, mice, and guinea pigs; avian pets such as parrots, passerines, and fowls; reptile pets such as turtles, alligators, crocodiles, lizards, and snakes; aquatic pets such as fish, freshwater snails, and saltwater snails; amphibians such as frogs and salamanders; 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 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. A Netherland Dwarf rabbit on a swing People most commonly get pets for companionship, to protect a home or property or because of the perceived beauty or attractiveness of the animals. Some scholars, ethicists, and animal rights organizations have raised concerns over keeping pets because of the lack of autonomy and the objectification of non-human animals.