Keep on doing things in your garden normally, and very once in a while you will sight a new type of piñata. When a new type of piñata enters your garden, move your cursor on it and press Y to see it's information. Look to the right in the information menu and it says what you have to do to make it a resident in your garden. Do those things and it should turn a different color and will be in your garden permanently. Just repeat that with 5 different species.