A leg is a common appendage of Humanoids and other animal species on Earth, and throughout outer space. Most species have at least two legs. Humans have two legs and use them for walking, running, jumping, kicking and a wide variety of other purposes.
When Doctor Rebos was putting his Wonder Dog android together, he asked Loco to hand him a leg, and Loco tells him he already handed him a leg. Then Rebos says: "Dumb, dumb, I'm not making a pogo stick! I mean another leg!