Wildlife Spotting

The horn is completely harmless, and actually fairly flexible. It has the feeling of a horn-like fingernail.

I have to disagree with both previous suggestions. To make a clear suggestion, one would need a photo of the side of the caterpillar. It is not the Tomato hornworm because it does not have a blue horn. The Great Ash Sphinx caterpillar doesn't have black diagonal lines. The lines are white and much more subtle. Do you have any other photos?

Great Ash Sphinx moth caterpillar

Yes it is a tomato caterpillar and what gives it away is the horn.