A baby. True it crawls on all fours, but cut its legs off and it can only wiggle around on two. Give it a crutch, and it can hobble around on three. You see?
Most notably known as the Sphinx's riddle. Four legs = baby at the dawn of life. Two legs = Man in the prime of his life Three legs = Old Man with a cane/walking stick in the twilight of his existence. Alright, that's your pebble of wisdom for the day.
Thats unfair, they should have slapped the Riddler. Its his joke. Of course, they would killed almost immediately, but...