LEGO Batman

Act 4 of Episode 2 in the Villain portion of the game puts you in control of Killer Croc and the Penguin. In the sewer tunnels, you will find pieces to begin construction of a large green crocodile ride-on (see the Achievement pic). You will need to smash several objects to unveil all the pieces. If your construct has a short stubby tail, you're missing some parts. Have you checked the three sewer grates in the wall in the left end of the tunnel?

Once the ride-on is completely built (and you can press Y to ride on it), the Achievement will unlock.