Kogitsunemaru was forged by Sanjou Munechika when he came to Fushimi. He met a mysterious boy near the Fushimi Inari gate who was believed to be a fox deity Inari when he prayed at the shrine before he forged the sword. The boy assisted him on forging the sword with his said to be inhuman skill and it turned out to be Munechikas great masterwork. He nicknamed it Kogitsunemaru kogitsune meaning little fox in honor of the deity that assisted him in forging the sword. Source: Touken Ranbu Wiki