It isn't unusual to find tobacco hornworm caterpillars on the datura plant.
What is unusual is to find one with such a long "horn." It is likely that it recently hatched from an egg.
Probably most of you know that insects need to shed their exoskeleton or outer "skin" in order to grow (as well as the linings of the breathing tubes and parts of their digestive system). When an immature insect sheds its exoskeleton or molts, it is said to enter the next "instar." The word instar is Latin and it means likeness or counterpart. The caterpillar above would be in its first instar.
Tobacco hornworm caterpillars generally go through 5 instars, but that number can vary with environmental conditions.
Before it pupates, the tobacco hornworm caterpillar will leave the plant and look for a place to burrow into the soil, as shown in this time lapse video.
Before long an adult moth will emerge and the cycle will continue.
Why do you think the first instar caterpillar is "rearing up" with its head away from the plant? Any ideas?