[One, Two, Middlesboogie]

Yes, they are born without legs. It'd be too costly to remove the legs of so many creatures; if that were the case it'd probably be better in the long run for the Vykkers to create a new servile creature (although knowing them they'd probably cock it up; they got the Fleeches very wrong).

If you look at some of the production sketches, you can see that the tail of a Slig is slightly lumpy, as though the tail and legs have merged into one prehensile limb throughout the course of evolution. Or maybe my imagination is just hyperactive.

EDIT: For the record, I like Glukkons. Or rather I did. I don't dislike them, but I'm just losing interest in Oddworld. I really can't bring myself to care one way or another.