In Wayne Barlowe's book Expedition he has some creatures who have become monopods, or only having one foot. They use it to hop through the dense air. Since the author never really got into the details , how do you see something like this evolving? If you have some other way monopods might evolve, that's allowed too, with one exception. No high gravity sliding on one foot like a snail creatures. That's already been done on deviant art and I want to encourage creativity.
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It is assumed, based on our scientific knowledge, that land animals have evolved from a species of fish which first ventured on the dry land in a distant past. Having evolved from this fish, all animals kept the structure with 4 limbs, deriving from the 4 fins of the fish.
Since you want to develop a monopode, look back at the fishes: do we have any fish which doesn't rely on 4 fins? YES!
Soles begin life as bilaterally symmetric larvae, with an eye on each side of the head, but during development, the left eye moves around onto the right side of the head. Adult soles lie on their left (blind) sides on the sea floor, often covered in mud, which in combination with their dark colours, makes them hard to spot.
Starting from this genus of fishes, organism might evolve leading to a motion like the one of some caterpillars, which use their body like a spring to perform jumps.
I know, the caterpillars technically have more than one foot, but I am using them as an example to convey the type of movement.