Content Paradise had a number of insects and arachnids as well as dinosaurs and crustaceans. You could just take that and create a head only from a wolf or dog from Daz or from the Poser set (the simplest is to just uncheck the 'visible' option on all the body parts) to parent to the scorpion's thorax. Sixus1 might have had a scorpian but I think they sell though Content Paradise now too.

BTW it is cool you have this vision of yours!

You can search freestuff with:
http://mystic-nights.com/directory/index.php

And commercial and freestuff with:
http://www.whatever3d.com/

whatever turned up theses scorpions...
http://www.daz3d.com/i.x/shop/itemdetails/-/?item=1337&spmeta=ap&refid=271299350
http://market.renderosity.com/mod/bcs/index.php?ViewProduct=8953&AID=448
http://www.contentparadise.com/us/user/spider_scorpion_product_11574
http://www.3dcommune.com/3d/store/more_info.mv?P1004304378

this wolf...
http://www.daz3d.com/i.x/shop/itemdetails/-/?item=342&spmeta=ap&refid=271299350
http://market.renderosity.com/mod/bcs/index.php?ViewProduct=31369&AID=448

Sorry I could not find freestuff but the wolf and dogs are part of some cheap content packages and maybe the Platinum reward packages from time to time if I recall correctly. There were some free wolves and dogs in the Poser 7 and Poser 4 content also if I remember.