These are children, almost always babies, who are taken and raised by wild animals. Occasionally they find their way back to society. They may be carried off from their villages by females to replace recently lost cubs, accidentally separated from their families during migrations or other movements, or possibly exposed in the wild as a form of infanticide/birth control. It is impossible to know, because the recovered children never learn to communicate with people and in any case they would have been taken by animals when they were too young to remember their real families.
Almost all examples are either fictional (such as Tarzan and Mowgli) or legendary (such as Afrasiab, King of Turan, in the Shah-nama; King Zahhak of Mesopotamia in the Avestas, Nebuchadnezzar II, Romulus and Remus, Cyrus the Great and Bear Woman), but there are a few fairly well-authenticated examples (see the references below) of apparently authentic feral children.
The host animals seem most often to be wolves, but bears, sheep, cattle, pigs, leopards and gazelles have also been recorded.
In contrast to fictional and legendary feral children, those in real life are completely unable to cope with human society and usually die soon after they are recovered from the wild, either from human illnesses to which they have no immunity, dietary shock or psychological trauma.
Some examples of probably authentic feral children are Kamala and Amala, discovered together aged about two and eight in Bengal in 1920, and Victor, the "Wild Boy of Aveyron" in France, who was first sighted in 1797 aged about nine and died in 1828. They were rescued and given into the care of responsible and caring professionals, but in spite of considerable efforts they never progressed very far in normal social development.
The earliest modern recorded case was the Wolf-Child of Hesse in 1344; the most recent seems to be the Ape-Child of Teheran in 1961.
Feral children are not the same as those tragic children raised in extreme isolation or locked in cupboards and cellars for years at a time by mentally disturbed parents. Nor are they the same as foundlings, whose mothers deliberately abandon them, but in circumstances where they know (or hope) they will soon be found and cared for.