This Hippo moves it's mouth up and down as it rolls along. There are two pegs on the front wheels that engage the mouth. The front wheels can be set to work together for slower chomping action, or opposite each other for faster chomping action. The eye pegs hold the head in place and allows it to move. Use the longer axle for the front wheels. You can rotate the wheels so the pegs are together or opposite during assembly. Use the shorter axle for the back wheels Print the head with supports to ensure a smooth action. Axle and eye peg diameter may need to be adjusted slightly depending on the resolution of your printer.
Based on a toy from David Wakefield in book How to Make Animated Toys