Mycah is a character in Game of Thrones, and is the son of a butcher. He is played by Rhodri Hosking and appears in the first season.
When he is encountered by Joffrey Baratheon practicing sword-fighting with Arya Stark, Joffrey confronts him and intends to kill the boy for 'pretending to be a knight'. He escapes the scene due to Joffrey's incapacitation at the hands (or jaws) of Nymeria but is later killed by the Hound under the orders of the prince.
In the books
In the Song of Ice and Fire novels, Mycah is thirteen years old. He falls in with King Robert Baratheon's party as it returns to King's Landing from Winterfell and befriends Arya Stark.