Skull Kid

In Ocarina of Time, the Skull Kid lived in the Lost Woods. It was said that when a Kokiri kid went into the Lost Woods and actually got lost, they would turn into a child of the forest known as a skull kid. Link befriended one of the Skull Kids by playing Saria's song for it.

Majora's Mask expanded on that same Skull Kid's story. Link had set out on a personal adventure, looking for an old friend, who actually was the Skull Kid. When he found him, the Skull Kid had been slightly corrupted by Majora's mask and didn't recognize Link as his old friend and stole his ocarina and Epona and fled into the land of Termina, but not before turning Link into a Deku scrub.