Your Wheel Pig is a Piggy Bank that fills up with diamonds you win over time. You collect diamonds for your Wheel Pig from gift boxes you win after the bonus round. The extra amount of diamonds you win depends on the box you received.
There's a small fee for opening up your Wheel Pig to collect the diamonds and you'll have to bank a minimum amount before you can break it open.
Your Wheel Pig becomes active once you've watched the tutorial, then you'll start collecting and saving!