Mukashi Horobita Maoujou de Hirotta Inu wa, Jitsu wa Densetsu no Majuu Deshita: Kakure Saikyoushoku "Hitsujikai" na Kizoku no Sannanbou, Izure, Hyaku Majuu no Ou to Naru

Is this "shepherd" out of the ordinary?! Alto, the third son of a noble family whose territory is the ruins of the Demon King's castle, got the job of "shepherd" when he turned 10 years old. "Shepherd" is said to be a weak job that is difficult to level up. One day, Alto picked up and saved a small dog in the Demon King's castle that he was cultivating. From the day he saved it without realizing that it was a legendary monster, Alto's level as a "shepherd" rose at an explosive rate! A "frontier gourmet fantasy" brought to you by the hidden strongest "shepherd" and the legendary sheep-herding monster.

(Source: Kodansha, translated)