Depending on where you live it might be some sort of Hawk Moth or a Fawn Sphinx. If you want to search for the Sphinx better enter "Sphinx kalmiae" otherwise most hits will be for the cat Sphinx. .
I actually saw a caterpillar that you are describing and did some researching. It is a Sphinx Caterpillar, commonly known as a Hornworm. Got to this link and scroll down a little to see a picture: http://www.simplegiftsfarm.com/caterpillar-identification.html#Caterpillar
Actually upon more research I found the Waved Sphinx here: http://www.whatsthiscaterpillar.co.uk/america/knobby.htm#horn
Color is a little different but it looks exactly like the one I found which sounds like yours too.
A large caterpillar that is green with yellow horns is the Cecropia Moth. These caterpillars get larger than others and can get up 4.5 inches in length.
Sphinx and many others of the horn worm species
A green hairy caterpillar is called a Automeris io. It will eventually turn into a silk moth and it has spines to help protect it.
The green caterpillar with the red horn is known as a horn worm. Since the horn is not a stinger, it is believed that the horn worm uses its horn as camouflage.
Lime Hawk-moth Mimas tiliae
The blue horned sperm worm
A caterpillar that is pink and as a blue horn and found in France is called a hawk moth. These moths can grow very large.
Not a Hawkmoth, this is a swallowtail butterfly caterpillar.
small and some can be green or blue or any color.
A horn worm either tomato or tobacco grows into a moth.
Lime Hawk moth catty
it is a Lime Hawk Moth originating from England
It's blue where it connects to the horn assembly, but else where it is yellow with green stripe.
The name of the lime green caterpillar with an orange head is the cecropia moth caterpillar. The entire body is lime green except for a few small parts on the head which are a bright orange color.
green prickly caterpillar
The horn relay in 1996 Geo Tracker is located under the left hand side of the instrument panel attached to the CEM. Two green wires connect the power from the fuse to the relay, a blue/green wire connects to the horn and a green/black wire carries power to the horn.