While everyone is probably thinking of Mowgli from The Jungle Book, it's actually quite terrible and sad. A "feral child" is a child that is raised from a very young age with little to no human contact. Because of this they have no experience with social skill or human language.
A 5 year old girl was found in Siberia who spent her entire life locked in a room with cats and dogs with no heat, water or bathroom. When found she couldn't speak Russian, would bark and jump as caretakers left and had "clear attributes of an animal." She's currently being cared for by psychologists in an orphanage.