The Kyrat Fashion Week quest to hunt the Mad Devil is located at KEO Gold Storage. The Mad Devil must be hunted with a bow or auto-cross, so consider taking the Recurve Bow or the auto-cross from the trading post with upgrades as you like. Its skin provides the final crafting upgrade for the Wallet, allowing 5,000,000 rupees to be carried. The Mad Devil can be found at these coordinates: 440, 815