Sir Jack Horner, or "Little Jack Horner" is the character from the nursery rhyme of the same name, about a little boy eating pie.
"Little Jack Horner / Sat in the corner / Eating a christmas pie. He put in his thumb / and pulled out a plum / and said 'What a good boy am I!'" 
In the Land of Stories series, "little Jack" has grown up and lives in the Red Riding Hood Kingdom (and presumably owns Jack Horner's Pie Shop in the village). He appears in A Grimm Warning, ch 6, as one of the Kingdom's representatives in the House of Progress.
Appearance and Personality
Jack Horner has clearly not abandoned his appetite for pies. He is described as an '"obese man with a guilty expression, eating pie".