Good to hear it works. Dark engine must be then ignoring missing joints or something.
Bugbeasts however DO have extra joints in arms and are a separate type of creatures from humanoids.
BTW. would you mind making the source file (.3ds ) public as well? I'd like to have a look at your setup. I opened your cobraman in 3dsmax using my tools and it's not getting right the leg setup, I think. It's most likely my tools get confused with a different layout of joints for the legs.
Also - I would suggest that you use the Constantine skeleton as a base, rather than humanoid. This is because Constantine has one more joint in legs, which would give more control over posing your Cobraman's tail. (one more bend, basically). But this would make sense, if you wanted to create new motions of course.