Hornworms are caterpillars that turn into a moth after pupation.
There are two similar species: the tobacco hornworm (with a red horn) and the tomato hornworm (with a black horn). Wild hornworms are pests, but a lot of hornworms are produced in captivity as pet food for various insectivores.

The adult moth form of Manduca sexta. This specimen has metamorphosed from an uneaten horn worm that has managed to hide inside a terrarium and pupate. Even though I have seen some claims on-line that hornworms fed only their chow cannot successfully pupate I now have to disagree.
While researching this article I have found several recipes how to cook hornworms for human consumption — the curious thing about them was that they weren’t food consumed in some exotic location, but North American ‘western’ food items made out of caterpillars.
Further Readings:
Tobacco Hornworm (Manduca sexta) – Encyclopedia of Life.
The Worm Lady Hornworms.
Manduca sexta on Wikipedia.
Manduca quinquemaculata on Wikipedia.
Fried Green Tomato Hornworms on The Daily Meal.
