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This big green caterpillar looks to be a Sphinx Moth caterpillar, Family Sphingidae. It's hard to say exactly which one it is. Do you have any other images, maybe of the side? Many of these guys look similar with slight differences in the stripes or color of the horn and such.
Try looking through these. You can refine the browse option to only see caterpillars, which would make it easier to look through images:
http://bugguide.net/node/view/39106/bgpa...
The caterpillar will be come a butterfly because it has smooth skin
A moth caterpillar would be fuzzy