There are several scifi creatures that have their locomotive appendages where we have our arms and their manipulation appendages where we have our legs. Look at Star Wars for an example. Why would any creature evolve this way? I can't figure it out.
Lots of possible reasons:
Since there aren't intelligent creatures to compare this arrangement to, the simplest answer is "because it's different and cool," but that's a cop-out (even if it's the true answer.) Let's come up with some possible reasons for this arrangement.
- Coincidence of evolution: Your creatures evolved from a species with hand-like appendages all round (like apes) but their ancestors resembled gorillas instead of bonobos. When evolving a stride for endurance hunting, their long-legged gait came from their existing long arms instead of their short back "feet." With time, the evolution of tool use and the minor role of their "feet" in walking meant the feet became hands and the hands became feet.
- Manipulative digits close to what they need to manipulate: Whatever this species needed to manipulate with dexterous hands was close to the ground. short "arms" close to the ground were efficient at using tools, digging, etc. At the same time, the upright stance of the creature let them be fully upright and alert to predators as they used tools close to the ground.
- Super arms: The creatures use their big arm-legs for major work, like crushing lion skulls or tearing down trees. These arms are immense from being used like legs, and your creature sits or uses it's small limbs to hobble around on when using the big arms. The arms are specialized for crude tasks, and for fine skilled tasks the delicate hand-feet are used.
- Endurance brachiation: Because of the way these critters move through a forest-like environment, they spend their time mostly bodily flinging themselves from tree to tree. Lots of apes can swing from vine to vine, but these guys make a full-time job of it. Their mass is well centered between their arms and the short legs are just there for an assist. There's the same justification for the legs becoming hands - they aren't being critical for motion, and are available to specialize into tool manipulators.
I'm sure there are more perfectly plausible reasons and I look forward to hearing what everyone has to say!
Its sensory organs are not directional.
With eyes, you need them in front to see where you are going and also what you are manipulating. If your strongest senses encompassed the entire area about you -- perhaps you use echo location -- it might be feasible to normally walk in the direction that leaves your manipulation appendages behind and so protected from what lies ahead of you.
However, you would still need to easily walk "backwards" in order to close with things that you want to manipulate.
The Caveman Coral Crab
In addition to being an alliterative animal, the Caveman Coral Crab has evolved in a strange way to fill its niche in the environment. While its front claws have shrunk in size comparatively to most crabs, they have become stronger and more adept at grabbing a hold of the coral reefs it exclusively lives in. Using these as a means of locomotion, they can scuttle around coral like paraplegic Spidermen on a NYC skyscraper .
Whenever it is time for a meal (or a predator is nearby), the Caveman crab can pull itself in to small crevices in the coral. If all is safe, it will allow its rear most appendages to wiggle in the water like sea worms. Whenever a curious fish comes by to take a nibble, the caveman crab's intermediate legs are thrust out, one set being spear like and the others wrap around the fish and pull it in to the crevice where the meal is consumed.
I think the crab is a fun place to start since several species have developed claws and legs in many different ways. Plus, animals in water give more leniency in using appendages for differing purposes.