In the wild I often see the large caterpillars feeding on willowherb and even sat on top of a bramble bush sunbathing. The full-grown caterpillar is very large for the size of the moth, sometimes green caterpillar can be found as full-grown larvae but usually, they soon change to brown/ black. It gets its name from a narrow trunk like front segment when alarmed it retracts its head which enlarges four-eyed spots to help deter predators.
In the wild the larvae can be found feeding on willowherb or bedstraw, if you open up a clump of willowherb sometimes you find many Elephant Hawk larvae feeding, probably from different moths. Larvae feeding high up on the plants have often been attacked by parasitic wasps. In urban areas they feed well on fuchsias which many people use in their gardens I once found full-grown larvae on my patio at midnight I realized it had fallen from my wife's hanging basket, and when I investigated I soon found others greedily feeding on the leaves and flowers.