This is probably not the best one out there but it is probably one of the smallest. It doesn't do much except crawl forward (or backwards). It can climb over some obstacles but not all. It is also easy and fast to make.
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The images show it movng forward.
Step 1: MainBody
Image 1: Make this.
Image 2: Place 2 white rods in the motor as shown.
Image 3: Connect the 2 together.
Step 2: Legs and Rest
Images 1-2: Make 2 of each.
Image 3: Put them together as shown.
Image 4: Add gray connectors as shown.
Image 5: Add a yellow rod and 2 spacers as shown to the motor.
Image 6: Connect the legs as shown.