I came across this very interesting looking caterpillar last weekend residing on our Grapevines.
It's not very long, about 3/4". Yellowish brown in color with a horn on it rear-end and what looks like an over sized (foreskin type) collar, which allowed it to retract it's head into.
It has since gone into it's chrysalis stage, so we'll see what type of butterfly emerges.
I'm guessing it's one of the following: Darapsa Myron, Achemon Sphinx or Pandorus Sphinx. Hard to tell as there too many to choose from, yet I wasn't able to locate an exact match on the Google machine.