feral child; wild child
Refers to a child raised in isolation from human contact, often in the wild, or metaphorically to a child with untamed, free-spirited behavior.
That novel is the story of a feral child raised in the forest.
He grew up in the city, but he's as free and uninhibited as a wild child.
Compound of 野生 (yasei, 'wild') and 児 (ji, 'child'). The term is a straightforward kanji compound with no irregular historical derivation.