I would guess it hatched on your plant from an egg laid by a moth.

Here is a link that might be useful: wiki article on hornworms

Next time you find one, check to see if it has, on its back, eggs the size and color of a grain of rice. Often times it will have several. If it does, and you can stand to do so, leave it be for a few days. That is the egg of a wasp. The egg hatches and then the larvae of the wasp devours the worm. Doing it this way ensure that the beneficial wasp lives to kill the next worm you miss.

Just to add........they grow into incredible hummingbird moths. Maybe plant an extra plant or two next year so you won't have to kill it.

I kill them all anyway. I use scissors.

Oh dear! I'd about cry if I came out to a tomato plant looking like that... I'd be breaking out the scissors, or my boot, or something to do it in. Wow.