So... I am no bug expert, but from what I could find it seems to be a Luna Moth Caterpillar, which turns into this beauty:
-Image compliments of Google-
They unfortunately don't inhabit my country, but they are quite a beautiful sight to behold. Nice find @gogogadgetupvote
I have never seen any butterflies or moths like that in the Pacific Northwest. What part of the world are those Luna Moth's from?
So... I am not from USA, so I do not know the geographic layout that well, but to borrow from Wikipedia...