Dorsidwarf, this one goes out for you.
You have, in your screature creature file, this:
[BODY_DETAIL_PLAN:BASIC_LAYERS_URD]
[BODY_DETAIL_PLAN:TAIL_LAYERS_URD]
[BODY_DETAIL_PLAN:LEG_LAYERS_URD]
This, under normal circumstances, would work. Normal circumstances being if the tissues you're trying to put on the creature are defined in the body detail plans, themselves. However, Urdothor set it up that way (and vanilla DF uses it similarly) that you can give it 'arguments', i.e., you can decide during the making of your creature what tissues you want on them.
So, for example, in your case, it would be something like this:
[BODY_DETAIL_PLAN:BASIC_LAYERS_URD:SKIN:FAT:MUSCLE:BONE:CARTILAGE]
[BODY_DETAIL_PLAN:TAIL_LAYERS_URD:SKIN:FAT:MUSCLE:BONE:CARTILAGE]
[BODY_DETAIL_PLAN:LEG_LAYERS_URD:SKIN:FAT:MUSCLE:BONE:CARTILAGE]
This would give the screatures cartilage, then bone, then muscle, then fat, then skin tissues, in that order.
Now, if you want to make 'em special, you can give them, for example, metal as bones, too. Simply do something like this:
[BODY_DETAIL_PLAN:BASIC_LAYERS_URD:SKIN:FAT:MUSCLE:IRON:IRON]
[BODY_DETAIL_PLAN:TAIL_LAYERS_URD:SKIN:FAT:MUSCLE:IRON:IRON]
[BODY_DETAIL_PLAN:LEG_LAYERS_URD:SKIN:FAT:MUSCLE:IRON:IRON]
This would replace all cartilage and bone with iron in your screature (though we have no iron in DFFS).
For now, I added cartilage, bone, muscle, fat and skin to your screatures, so that they don't spam my errorlog, but feel free to play around with it. Post any changes you want made.
I'm not sure how I missed this and how nobody helped you on this, but here you go now. Slap me with your freshest scraw RAW and we can have that in, too.