You can receive the special skin for your panda by completing all tasks in the "Legends of the Water Valleys" special event. You can find the tasks in the bottom-left part of the screen.
You need to collect all books, fix all wells, and provide water for the llamas. You can find them exploring the event maps. The objectives of special tasks are hidden in the fog. You need to clear the location to find llamas, wells and ancient books.