Viva Piñata

To name a piñata you must first highlight them with your cursor and then press the "Y" button, this will bring up the information concerning that particular piñata, while in that menu use the D-pad to select the "information" tab and then press "A", this will allow you to rename the piñata, once you've named the piñata and finished doing so the achievement will unlock!

Put your cursor on a Piñata and hit the Y button to view statistics about your Piñata. Move down once to information and there you can press A to name the Piñata.