The mythical King of Beer.
While there are several stories behind Gambrinus, he seems to be a hodgepodge of actual historical figures: Jan Primus and Jean sans Peur. Jan Primus of Brabant lived from 1251 to 1295 and was a Burgundian duke who served as "king" of the Brussels brewers guild. Jean sans Peur - John the Fearless - lived from 1371 to 1419 and is sometimes referred to as the inventor of hopped beer.
Jan Gambrinus is typically pictured riding a giant cask and wearing an ermine coat, a crown, and a giant mug of ale.