LEGO MINDSTORMS Robot Inventor
This model shows a simple way to make a 4-leg walker by using a motor to control each leg. It walks with an alternating left/right and front/back pattern similar to an animal.
Although it uses all of the motors in the kit, you can add non-moving parts to it to make various animals that you can invent, such as this Spider.
If you need spare motors and don't need the robot to turn, you could reduce it to two motors by grouping the front and rear legs into pairs driven by one motor as in this Cat, or with more work you can even do it with only one motor as shown in the 1-Motor Walker.