Skull Kid is a character from the Nintendo 64 game The Legend of Zelda: Ocarina of Time (a non-significant appearance) and notably The Legend of Zelda: Majora's Mask. The alternate spelling of this is Skullkid, and is the spelling used in the game. In manga and in early translations, his name is roughly translated as Star Kid, coming from the name Sutaru Kiddo (スタルキッド), since the Japanese langauge spells foreign words based on phonetics rather than spelling. This was, however, an error; the character is known as "Stalkid" in Japan, based on the traditional Zelda skeleton enemy known as Stalfos.
Ocarina Of Time
Skull Kid makes a brief appearance in Ocarina of Time. He can be found in the Lost Woods dancing on a tree stump. The player can teach him Saria's Song on the ocarina. When completing the side quest, the Skull mask is given to him. Also, later in the game, after you receive the Biggoron Sword, if you return to the Lost Woods as an adult and kill the Skull Kid you will receive a giant rupee worth 500 rupees. On a side note, his past is shrouded in mystery, and no one knows where he came from. Some say he is simply a forest sprite, but the Ocarina of Time manga states that while his family was on a picnic, a young child ventured into the Lost Woods, deaf to his mother's warnings. He wanted to catch a fairy, but instead got lost. After the fear subsided, he breathed in the dark air easily, and was gradually transformed into a Skull Kid, the final form that befalls Hylian children or Kokiri who venture into the Lost Woods. (It is seen in the manga and in the game that there are more than just one Skull Kid.)
In Majora's Mask, his two faerie sidekicks Tatl and Tael scare the horse Epona, which causes Link to be thrown off and knocked unconscious. He comes and pats down Link, looking for anything valuable, settling for the Ocarina of Time, which he begins to blow into, waking up Link. He lunges for Skullkid, but he hops on Epona for his getaway. After holding on a while, Link falls off, and follows him into a hollow log and falls into an abyss (disputedly where you travel to Termina). Then Skull Kid turns Link into a Deku Scrub and leaves. He tries to make the moon fall on Termina and the player finds out later that Majora's Mask is controlling him.