The only origin account Wikipedia gives is a legend that the name comes from a cook called Angiolino, or "Angelot", from Montferrat, said to be the inventor of the recipe; no date or period is attached to this legend and the article gives no other historical dating for when agnolotti took shape.
What would resolve this A food-history source establishing when stuffed pasta of this square-folded Piedmont form was first made or documented, independent of the undated Angiolino naming legend.
Openfood historyAgnolotti (Wikipedia)