This quest becomes available during the Stop Ranrok and Rookwood main objective. Travel to Hogsmeade then head for the bridge to the left of J. Pippin’s Potions. Speak with Garnuff under the bridge to learn his precious Mooncalf was stolen by poachers.
Exit Hogsmeade and head north past Upper Hogsfield to find the poacher’s camp. Defeat the lot of them then open the cage to free Biscuit and the other Mooncalfs. You’ll then need to rescue Biscuit using your bag. Also, make sure to check the nearby tent to find a collection chest.
Return to Garnuff to complete the quest. You can choose to hand Biscuit over or convince the Goblin she’ll be safe with you. You’ll receive the Beast Rescuer Robe appearance as a reward.