Although there are no caterpillars called the green horned-butt caterpillar, except maybe colloquially, two certainly fit the description. Both the tomato hornworm (Manduca quinquemaculata) and the tobacco hornworm (Manduca sexta) are both green and have horned butts. Gardeners tend not to be fans of these species, as they can cause significant damage to tomato, potato, eggplant and pepper plants. Once you know how to identify them, they're quite easy to spot -- although the differences between the two are subtle.
Watching For Signs
Before you see tomato or tobacco hornworms, you might notice signs of their presence. You can identify them by the large, black droppings, known as frass, that will build up on the ground underneath or on the leave of affected plants. You may also notice the foliage on your plants becoming seriously wilted or simply disappearing.
Color and Markings
Tomato and tobacco hornworms are bright green in color, which can make them hard to spot among the green foliage on which they live. If you spot a green caterpillar, look closely at its markings to determine whether it might be one of these caterpillars. Tomato hornworms have eight V-shaped markings along their torsos, either white or pale yellow. Tobacco hornworms have similar markings, but they're subtly different: the eight white or pale yellow stripes along their torsos are diagonal.
The name hornworm comes from the distinctive horn that these specimens have on their rear ends. Tomato hornworms' horns are either dark blue, dark green or black, and are relatively straight. Tobacco hornworms, on the other hand, have red horns on their rear ends that are more curved in shape.
Another way you might identify tomato and tobacco hornworms is by size. No diminutive caterpillars, members of these species can reach up to 5 inches in length once they're fully grown. This is rare among caterpillars, so if you see any extremely large specimens, they're likely to be either tomato or tobacco hornworms.