The name Herpin does not derive from a particular region in France. After my research I can say that between 1100 and 1700 it is found numerously in the west of Bourges along the Loire and its tributaries.
Harpain derives from harpoon. Harpain means “master of bread”. These are often heard interpretations. But both are wrong. According to Albert Dauzat(1) Herpin means Harp player. This interpretation can be followed back to the year 1380 at least. Jehan de Vielbourg(2), an officer of the Duke of Berry, is called Harpin, harp player. His children had the name Herpin.
Herpin and Harpin result from vocal transposition. Herpin and Herpain respectively Harpin and Harpain are pronounced the same in French, the H is not spelled.
(1) Albert Dauzat, Dictionaire Etymologique des Noms de Famille et Prénoms de France, Librairie Larousse, Paris 1951
(2) www.clauderioland.net/berry/poulaines/chateaupoul.htm