Big Yan is the biggest Pictsie (a giant of 8") on the Chalk. He was tough enough to knock out a horse in two headbutts. He is one of the more senior of Rob Anybody's brothers. Big Yan is a reference to Glaswegian comedian Billy Connolly who is known as "The Big Yin". Yan in the 'Yan, Tan, Tethera" counting system of Scotland and Northumbria meaning "one". In the Discworld it is "Yin, Tan, Tethera".
Since the Pictsies are based on Celtic Scots, Yan could also be a variation on 'Ian" ('I' and 'J' were once the same letter) the Gaelic version of the name "John". Although large by Feegle standards, he's still small by human standards, so he'd be a very small Big John. According to legend, one of Robin Hood's most famous outlaws was an extremely tall man called Little John. Whether Pratchett had the Robin Hood legend in mind is unknown. He certainly enjoyed playing with the Pictsies size within their names; Wee Jock, Big Wee Jock, etc.