[Munch's Master]

The tails could have once been legs. I think it's possible that the tail was origially a single leg designed for them to hop on (ala Gabbits and Meeps) but as they evolved they couldn't bear their bodies' weight on the leg so it became more atrophied and turned into a tail. I still like the tree dwellers theory, but if they do come from trees I doubt Vykkers do as the 2 species would most likely have fought each other.