INTP:
As a supposed engineering student, you might first imagine something as simple as a motor and a cam system. 2 DC motors with nubs on the shaft are all that are needed. The toys move very obviously like cars that are divided left side and right side wheel control.
The rest of you folks surprise me. Tsk.
Hey were are all thinking outside the circle... (Or is it square? ? ?).... :o 
The leg movement is more like;
Front and Back on left side working with Centre on right side.
Front and Back on right side working with Centre on left side.
Tom..