I actually think that Wreck-gar is a great mold choice, obakemono. When I think of Taratulus I think of motorcycles due to his transmetal mode and the body shape is actually pretty similar to Taranchulus' transmetal robot mode. Heck, find a way to parts form the wheel leg to go on his other shoulder and he would be pretty spot on.
I'm liking the idea more and more now that I think about it. I may have to try making one myself someday.
If you're still looking for ideas, though, General Techno already suggested the DOTM Backfire mold.
http://www.tfu.info/2011/Autobot/Backfire/backfire.htm
He kinda does evokes a Tarantulus look with his head, alt mode, and body features. His chest has details that could be used as spider eyes and whatnot.
