OK option 2 would be to try and use a skeliton from one of the figures that is already rigged, for this all you would need is say a V3, M3, G2 or alike figure that has the bones you need for your creature.
**
Note: This Might Not Work but is Worth A Try**

Step 1, load your creature into Poser
Step 2. click on the Setup Room tab and click OK on the warning dialog box.
Step 3. find the doner figure in the librery and load it.
Step 4. Tweek the bones to better fit your figure, you can delete or add any extra bones as or if needed.
Step 5. Open the Grouping Tool
Step 6. Click the Auto Group button
Step 7. Close the Grouping Tool and Exit the Setup Room
Step 8. The Moment of Truth, If all goes well you should now be able to pose your creature.
Step 9. Save your working Creature to your folder in the Figures Librery.

As I say this way might or mightnot work, good luck.
Steve