I'm thinking a skeleton of something, a butterfly with a torn wing, maybe even a giant lizard or a baby turtle... I turn around and OMGWTFHOLYBEJEEZUS PUT THAT THING DOWN!!
Bright green, the size of my finger, covered in hair and spiky horns with an intimidating shiny yellow face and pointy fangs and pincers, I was pretty shaken.
I ran in circles for a minute, then grabbed a shoebox to contain worm-zilla, and calmed down long enough to google our find.
Before long my closest guess-timate is that we are the lucky discoverers of a young:
According to the sources I found, they are harmless and the larval state of the Regal Moth. The moth only has one generation of offspring per year, and unlike other caterpillars that spin a cocoon, this caterpillar burrows into the ground for pupation.
We basically took a chunk out of the ground he was found upon and put it into an aquarium with his favorite vegetation so that he can be his sweet little wormy self without threat of being eaten or stomped by something outdoors. God, he's cute, and he eats and poops a lot. *snuggles my worm*
Source : My Backyard :-)
Edit : According to oakenking, I was wrong. Apparently I have an "Imperial Moth Larva-Eacles imperialis". So.. two for one! Learn about the Hickory Horned Devil Caterpillar AND the Imperial Caterpillar, both made of awesome.